Hi,
I had a similar behavior some days before. That damn gereric rtk NIC
felt more like a 56kp/s modem, than a fast-ethernet card. After fixing it
to 100Mb/s half-duplex mode, problems went away. Maybe this can help
you too.

Whith "netstat -s" on a DOS-Box you can take a look on built-in packet 
statisics. Somewhere odd error counts?

And there is this (in)famous "MTU to big and path MTU detection failed" 
issue, which is always capable to mess things up. How to work around this
with MMS clamping on the gateway, is discribed in the pppoe(4) man page.
Or you set it down by hand on you windows host with tools like Dr. TCP.

Ok, this does not explain why SSH'ing to the NetBSD host (It is on your 
LAN isn't it?) also get's messed up but I ran out of ideas now ;-)

HTH

-Alex