It's frustrating, but, if you've followed the advice given so far, I have a feeling it's a hardware issue. Random restarts would hint to it being a processor issue. Is it an ATI HD graphics card? Can you check your system log and search the HDD for a .dmp file?
It's an nVidia Geforce 6800 xt i believe, my processor is very poor tho, the cpu on the gfx card probobly runs better LOL , it's a 2.1ghz processor, but it's pretty clogged with dust and shit, and i haven't had time to clean it.