> after a netperf run, we have:
> 0  1500  <Link>        00:a0:c9:e4:63:95     5708     0    14567     1   141
> 
> so 41 more collisions, for an additional 660 or so Ipkts, and 4000 or so 
> Opkts. 

Note: An interface in full-duplex mode doesn't do collision detection
(although it's possible it may detect a collision jam pattern from
another host).  Clearly there is something wrong here.

> The machine with the de0 shows the following:
> Name  Mtu   Network       Address              Ipkts Ierrs    Opkts 
> Oerrs  Coll
> de0   1500  <Link>        00:c0:f0:31:61:5a   115046   177    90353     
> 3     0
> de0   1500  130.33.112/21 130.33.113.32       115046   177    90353     
> 3     0
> 
> so O(100K) packets, 0 collisions, 177 Ierrs. Things seem to work right, 
> just very slowly. 

The large number of ierrs is indicative of late or undetected
collisions.

I think it's pretty clear that your fxp0 is not in full-duplex mode.

I'd suggest you compile a new ifconfig(8) first.  What you should see
from ifconfig is:

# ifconfig fxp0
fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        media: Ethernet 100baseTX full-duplex
        status: active
        inet 24.128.118.236 netmask 0xfffffe00 broadcast 24.128.119.255
# 

If your interface is in the wrong mode, you'll see:

# ifconfig fxp0
fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        media: Ethernet 100baseTX
        status: active
        inet 24.128.118.236 netmask 0xfffffe00 broadcast 24.128.119.255
# 

You can change it by doing:

# ifconfig fxp0 media 100baseTX mediaopt full-duplex