well, sounds like you're beyond the basics and you've checked your drivers... so i'll go with hardware...

check your power supply... is it enough?
next, i'd guess the memory is hosed. got any way to check it? how about just swapping it out?
could your motherboard be hosed? i had a 750i SLI FTW that was whacked... changed it out and all was better (i was getting hard system lockups)

let me know if that helps.

Rob