Got lots of these on the console:

ahc0: target 1 synchronous at 10.0MHz, offset = 0xf

Every now and then, there scrolled a message like this:

ahc0: target 2 synchronous at 10.0MHz, offset = 0xf
sd1(ahc0:1:0): timed out in message out phase, SCSISIGI == 0xb6
ahc0: Issued Channel A Bus Reset #1. 8 SCBs aborted
ahc0: target 1 synchronous at 10.0MHz, offset = 0xf

Or like this:

ahc0: target 1 synchronous at 10.0MHz, offset = 0xf
sd0(ahc0:0:0): timed out in message out phase, SCSISIGI == 0xe6
sd1(ahc0:1:0): asserted ATN - device reset in message buffer

This kept going all the time and the system was unusable.  This started
while the machine was running (not during reboot).

Here's the ahc related dmesg output:

ahc0 at pci0 dev 20 function 0
ahc0: interrupting at irq 11
ahc0: aic7880 Wide Channel, SCSI Id=7, 16 SCBs
scsibus0 at ahc0 channel 0: 16 targets
ahc0: target 0 synchronous at 10.0MHz, offset = 0xf
sd0 at scsibus0 targ 0 lun 0: <SEAGATE, ST15150N, 0017> SCSI2 0/direct fixed
sd0: 4095MB, 3712 cyl, 21 head, 107 sec, 512 bytes/sect x 8388315 sectors
ahc0: target 1 synchronous at 10.0MHz, offset = 0xf
sd1 at scsibus0 targ 1 lun 0: <SEAGATE, ST34371N, 0280> SCSI2 0/direct fixed
sd1: 4148MB, 5168 cyl, 10 head, 164 sec, 512 bytes/sect x 8496960 sectors
ahc0: target 2 synchronous at 10.0MHz, offset = 0xf
sd2 at scsibus0 targ 2 lun 0: <SEAGATE, ST34371N, 0280> SCSI2 0/direct fixed
sd2: 4148MB, 5168 cyl, 10 head, 164 sec, 512 bytes/sect x 8496960 sectors
ahc0: target 4 synchronous at 5.0MHz, offset = 0xf
st0 at scsibus0 targ 4 lun 0: <DEC, DLT2000, 971E> SCSI2 1/sequential removable
st0: drive empty

We see the same problem on several machines every now and then, so I think
it's not a hardware problem.  Any ideas?

  -jm