PPP maybe? I am running two Linux boxes which are sometimes quite
heavily loaded, but they are Ethernet-only (no PPP), with SMC8013 and
WD8003-old, IDE and ESDI hard drives, and I see no crashes (although
there were some problems with late 1.1.x and early 1.2.x kernels).
One old 386 running 1.2.11 was up for only 78 days because of power
failure :-). I have heard some stories about PPP problems. The
"kernel" is quite big, try to narrow down the problem...
I've used PPP fairly heavily under various kernels between 1.3.15 and
1.3.27 with no problems. I've never left it up for more than 10 hours
or so, but I've never had it crash, either.