Wow, no wonder nobody replied yet.
I haven't had an issue that bad, but the fact that you don't really know what brand of motherboard makes me suspicious. You could try replacing it, it's probably not that expensive (get an ASUS) but it could also be the RAM. I think it's your mobo, though. Make sure you have the right drivers for everything.
Is the ethernet connecter for the internet on the motherboard or on a separate PCI card?
Asus P4P 800-E Deluxe, Intel P4 3.0 GHz
1GB RAM, ATI Radeon X1300 pro 256 mb
20GB maxtor, 250GB western digital