I can't point my finger on the date that this started exactly happening,
but kernels compiled back in May worked fine and after the 12th of June
they seem to work less fine. Here's the output:

root on sd1a dumps on sd0b                         
data fault: pc=0xf019ae2c addr=0x3a2061b0 ser=80<INVAL>
panic: kernel fault                                    
Begin traceback... 
normal_mem_fault(0xf01be710, 0x3a206174, 0xf0002000, 0xf02164c0, 0xffffffff, 0x3) at normal_mem_fault+0x28
ltsleep(0xf01f5ac4, 0xf01be710, 0x400, 0xf1a474f8, 0x20000000, 0xf0075980) at ltsleep+0x1e4
softdep_process_worklist(0x0, 0xf019b284, 0xf0212400, 0xf01e9400, 0xf0212400, 0xf0212400) at softdep_process_worklist+0xb4
sched_sync(0xf1a474f8, 0x30000000, 0x3949fe82, 0xf01f6000, 0xf0217400, 0xf0214400) at sched_sync+0x244
proc_trampoline(0x0, 0x0, 0x0, 0x0, 0x0, 0x0) at proc_trampoline
End traceback...                                                
syncing disks... done
Frame pointer is at 0xf1a55b68
Call traceback:               
  pc = 0xf0180ad4  args = (0x110000e1, 0x11000fe1, 0x0, 0x9c40, 0xf01b1400, 0xf01e7000, 0xf1a55bd0) fp = 0xf1a55bd0
  pc = 0xf0042cf4  args = (0x100, 0x0, 0xfffa, 0xf1a55f70, 0xf00433e0, 0x119000e2, 0xf1a55c38) fp = 0xf1a55c38
  pc = 0xf0188a28  args = (0xf01c5440, 0x1, 0x100, 0xf1a55d00, 0x37, 0x7, 0xf1a55ca0) fp = 0xf1a55ca0
  pc = 0xf00062f4  args = (0x9, 0x80, 0x3a2061b0,~0xf019ae2c, 0x40, 0xf1a55da8, 0xf1a55d48) fp = 0xf1a55d48
  pc = 0xf003a624  args = (0xf01be710, 0x3a206174, 0xf0002000, 0xf02164c0, 0xffffffff, 0x3, 0xf1a55df8) fp = 0xf1a55df8
  pc = 0xf019b338  args = (0xf01f5ac4, 0xf01be710, 0x400, 0xf1a474f8, 0x20000000, 0xf0075980, 0xf1a55e60) fp = 0xf1a55e60
  pc = 0xf0075bc4  args = (0x0, 0xf019b284, 0xf0212400, 0xf01e9400, 0xf0212400, 0xf0212400, 0xf1a55ec8) fp = 0xf1a55ec8
  pc = 0xf0007ac8  args = (0xf1a474f8, 0x30000000, 0x3949fe82, 0xf01f6000, 0xf0217400, 0xf0214400, 0xf1a55f50) fp = 0xf1a55f50
  pc = 0x0  args = (0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) fp = 0x0
                                                               
dumping to dev 7,1 offset 147595
Stopped in ioflush at   cpu_Debugger+0x4:       jmpl            [%o7 + 0x8], %g0

And my kernel config file:

---SNIP---
include "arch/sparc/conf/std.sparc"

#ident 		"GENERIC-$Revision: 1.105 $"

maxusers	32

## System kernel configuration.  See options(4) for more detail.


# Options for variants of the Sun SPARC architecure.
# We currently support three architecture types; at least one is required.
options 	SUN4C		# sun4c - SS1, 1+, 2, ELC, SLC, IPC, IPX, etc.

## System options specific to the sparc machine type

# Blink the power LED on some machines to indicate the system load.
options 	BLINK

## Use a faster console than the PROM's slow drawing routines.  Not needed
## for headless (no framebuffer) machines.
options 	RASTERCONSOLE		# fast rasterop console
options 	FONT_GALLANT12x22	# the console font
#options 	FONT_BOLD8x16		# a somewhat smaller font
## default console colors: black-on-white; this can be changed
## using the following two options.
#options 	RASTERCONSOLE_FGCOL=WSCOL_WHITE
#options 	RASTERCONSOLE_BGCOL=WSCOL_BLACK

#### System options that are the same for all ports

## Root device configuration: change the ?'s if you are going to use a
## nonstandard root partition (other than where the kernel is booted from)
## and/or nonstandard root type (not ffs or nfs).  Normally this can be
## automagically determined at boot time.

config		netbsd	root on sd1a type ffs dumps on sd0b

## System call tracing (see ktrace(1)).
options 	KTRACE

## System V compatible IPC subsystem.  (msgctl(2), semctl(2), and shmctl(2))
options 	SYSVMSG		# System V message queues
options 	SYSVSEM		# System V semaphores
options 	SYSVSHM		# System V shared memory
#options 	SHMMAXPGS=1024	# 1024 pages is the default

#### Debugging options

## The DDB in-kernel debugger runs at panic (unless DDB_ONPANIC=0), or at
## serial console break or keyboard reset, where the PROM would normally
## intercept.  DDB_HISTORY_SIZE adds up/down arrow command history.
options 	DDB			# kernel dynamic debugger
options 	DDB_HISTORY_SIZE=100	# enable history editing in DDB
options 	DDB_ONPANIC=0		# see also sysctl(8): `ddb.onpanic'


## Compile the kernel with debugging symbols (`netbsd.gdb' is the debug file),
## such that gdb(1) can be used on a kernel coredump.

makeoptions 	DEBUG="-g"


## Make SCSI error messages more verbose when explaining their meanings.
options 	SCSIVERBOSE


## Options for compatibility with previous releases foreign system binaries.
## In the cases of COMPAT_SUNOS and COMPAT_SVR4, you may need to set up
## additional user-level utilities or system configuration files. See
## compat_sunos(8) and compat_svr4(8).

options 	COMPAT_43	# 4.3BSD system interfaces
options 	COMPAT_10	# NetBSD 1.0 binary compatibility
options 	COMPAT_11	# NetBSD 1.1 binary compatibility
options 	COMPAT_12	# NetBSD 1.2 binary compatibility
options 	COMPAT_13	# NetBSD 1.3 binary compatibility
options 	COMPAT_14	# NetBSD 1.4 binary compatibility
options 	COMPAT_SUNOS	# SunOS 4.x binary compatibility
options 	COMPAT_SVR4	# SunOS 5.x binary compatibility
options 	COMPAT_AOUT	# NetBSD a.out compatibility

## File systems.  You probably need at least one of FFS or NFS.
file-system	FFS		# Berkeley Fast Filesystem
file-system	NFS		# Sun NFS-compatible filesystem client
file-system	KERNFS		# kernel data-structure filesystem
file-system	NULLFS		# NULL layered filesystem
file-system 	OVERLAY		# overlay file system
file-system	MFS		# memory-based filesystem
file-system	FDESC		# user file descriptor filesystem
file-system	UMAPFS		# uid/gid remapping filesystem
file-system	LFS		# Log-based filesystem (still experimental)
#file-system	PORTAL		# portal filesystem (still experimental)
file-system	PROCFS		# /proc
file-system	CD9660		# ISO 9660 + Rock Ridge file system
#file-system	UNION		# union file system
#file-system	MSDOSFS		# MS-DOS FAT filesystem(s).
#file-system	CODA		# Coda File System; also needs vcoda (below)

## File system options.
options 	NFSSERVER	# Sun NFS-compatible filesystem server
#options 	QUOTA		# FFS quotas
#options 	FFS_EI		# FFS Endian Independent support
options 	SOFTDEP         # FFS soft updates support.

## Network protocol support.  In most environments, INET is required.
options 	INET		# IP (Internet Protocol) v4
options 	INET6		# IPV6
options 	IPSEC		# IP security
options 	IPSEC_ESP	# IP security (encryption part; define w/IPSEC)
#options 	IPSEC_DEBUG	# debug for IP security
#options 	GATEWAY		# packet forwarding ("router switch")
#options 	MROUTING	# packet forwarding of multicast packets
#options 	DIRECTED_BROADCAST	# allow broadcasts through routers
#options 	NS		# Xerox NS networking
#options 	NSIP		# Xerox NS tunneling over IP
#options 	ISO,TPIP	# OSI networking
#options 	EON		# OSI tunneling over IP
#options 	CCITT,LLC,HDLC	# X.25 packet switched protocol
#options 	NETATALK	# AppleTalk (over Ethernet) protocol
options 	NTP		# Network Time Protocol in-kernel support
#options 	PPS_SYNC	# Add serial line synchronization for NTP
options 	PFIL_HOOKS	# Add pfil(9) hooks, intended for custom LKMs.
options 	IPFILTER_LOG	# Add ipmon(8) logging for ipfilter device
#options 	PPP_BSDCOMP	# Add BSD compression to ppp device
#options 	PPP_DEFLATE	# Add deflate (libz) compression to ppp device
#options 	PPP_FILTER	# Add active filters for ppp (via bpf)



#### Main bus and CPU .. all systems.
mainbus0 at root
cpu0	at mainbus0

#### Bus types found on SPARC systems.

sbus0	at mainbus0				# sun4c

#### Standard system devices -- all required for a given architecture

## Auxiliary system registers on sun4c and sun4m
auxreg0	at mainbus0				# sun4c


## Mostek clock found on 4/300, sun4c, and sun4m systems.
## The Mostek clock NVRAM is the "eeprom" on sun4/300 systems.
clock0	at mainbus0				# sun4c


## Memory error registers.
memreg0	at mainbus0				# sun4c

## Timer chip found on 4/300, sun4c, and sun4m systems.
timer0	at mainbus0				# sun4c


#### Serial port configuration

## Zilog 8530 serial chips.  Each has two-channels.
## zs0 is ttya and ttyb.  zs1 is the keyboard and mouse.
zs0	at mainbus0					# sun4c
zstty0	at zs0 channel 0				# ttya
zstty1	at zs0 channel 1				# ttyb

zs1	at mainbus0					# sun4c
kbd0	at zs1 channel 0				# keyboard
ms0	at zs1 channel 1				# mouse

#### Disk controllers and disks

#

## The following flags may be set for the NCR53c94 based esp driver:
##	bits 0-7:  disable disconnect/reselect for the corresponding target
##	bits 8-15: disable synchronous negotiation for target [bit-8]

## sun4/300, sun4c, sun4m on-board SCSI, and FSBE/S SBus SCSI cards.
## Both `dma' and `esp' are needed in all cases.
## Two kinds of additional SBus SCSI interfaces are available.  One uses
## "esp at sbus" like the sun4c on-board; the other uses "esp at dma".

## sun4/300 SCSI - an NCR53c94 or equivalent behind
## an LSI Logic DMA controller

dma0	at sbus0 slot ? offset ?			# sun4c/sun4m
esp0	at sbus0 slot ? offset ? flags 0x0000		# sun4c

scsibus* at esp?

## These entries find devices on all SCSI busses and assign
## unit numbers dynamically.
sd*	at scsibus? target ? lun ?		# SCSI disks
st*	at scsibus? target ? lun ?		# SCSI tapes
cd*	at scsibus? target ? lun ?		# SCSI CD-ROMs
#ch*	at scsibus? target ? lun ?		# SCSI changer devices
#ss*	at scsibus? target ? lun ?		# SCSI scanners
#uk*	at scsibus? target ? lun ?		# unknown SCSI


## Floppy controller and drive found on SPARCstations.

fdc0	at mainbus0				# sun4c controller
fd*	at fdc0					# the drive itself




#### Network interfaces

## LANCE Ethernet - an AMD 7990 LANCE behind specialized DMA glue
## Three flavors of additional SBus ethernets are available.  One attaches
## directly like the sun4c on-board, one uses the ledma device like the
## sun4m on-board, and one uses the lebuffer device.

le0		at sbus0 slot ? offset ?		# sun4c on-board


#### Audio and video devices

## /dev/audio support (`audioamd' plus `audio')
##
audioamd0	at mainbus0				# sun4c
audio*		at audioamd0


## Sun "cgsix" accelerated color framebuffer.
cgsix0		at sbus? slot ? offset ?


## pseudo-devices

pseudo-device	pty		32	# pseudo-ttys (for network, etc.)
pseudo-device	rnd
pseudo-device	vnd		4
pseudo-device	md		1
pseudo-device	loop
pseudo-device	ipfilter
pseudo-device	bpfilter	8
pseudo-device	gif		4	# IPv[46] over IPv[46] tunnel (RFC1933)
---SNIP---

Has anyone noticed anything similar, on even has a gut feeling as to
where to start looking for the problem?

Thanks,
  Antti

-- 
Antti Kantee <pooka@iki.fi>                    Of course he runs NetBSD.