[NTLUG:Discuss] Network saturation?

[Brian]

Scenario: Box 1 is a stock RH6.1 install on a Dell Latitude laptop.  Box
2 is a highly-modified SuSE 6.1 install.  Both running 2.2.x kernels. 
Linked via 10BaseT hub, no other nodes.

Just after both machines are booted up, network throughput between the
two machines is somewhere around 1MB/sec.  After a few hours of network
activity and whatnot, throughput *drastically* drops to about 2KB/sec. 
netstat shows nothing out of the ordinary in the way of dropped packets,
etc.  Surprisingly, a 'shutdown -r' on both machines fails to correct
the problem.  Throughput on the SuSE box outbound via a ppp connection
is normal under all circumstances.

Where do I start to look to diagnose this?