Network jams on RTL8139s

Donald Becker,
This is more an offer of help for you than yet another moan!
I've got an rtl8139b which I was trying to run under your latest drivers -
see earlier correspondance -
which I eventually replaced with an SMC9432 - the first card didn't work -
which is now working fine.
I can easily reproduce the network hanging bug that rtl's seem to have on
linux (just by starting an x-client on a remote machine). In thanks for your
for your efforts I am offering to have a go at debugging the problem - have
you got any tips for carrying out this operation?
The bug I'm talking about is the one where the network runs for a while and
then dissappears under heavy traffic requiring either a reboot or an ifdown
/ ifup to get it going again.
Hope this makes sense...
Bill