Have you tried leaving you modem unhooked? Have you tried the other ethernet port on your mb? I've never seen anything like PC restarts be associated with a modem.
I think in Asus' bios there is a way to disable each ethernet port. I know in my P5K you can disable either one or both.
I doubt the ethernet has anything to do with it though and it does sound like a stability issue to me. Do you have NB/SB cooling other than stock? My P5N32-E SLI 680i didn't like a NB over 1.35 with stock cooling, not sure why.
I would lower that SB it shouldn't be that high for that overclock. Only other thing I can say is your NB might need to be raised slightly, or lowered. If lowered doesn't help then I would say you need some other cooling on it and to raise it.
What I noticed, was that when I start BitComet and start downloading, after a while my modem restarts..and then my pc. No BitCOmet is closed and I'm not downloading, and didn't had a restart yet.