The best thing to do is to start swapping out parts with another computer and eliminate possible sources of the problem. I have 4 computers at home, which is very useful for testing things out.
I had a similar problem several years ago with an ASUS motherboard using the original nForce chipset. I upgraded my video card to an MSI 6600GT (from a Ti4200), and I would get weird graphical glitches and very erratic behavior - while sometimes it would work for a few days just fine. I tried several driver revisions, updated all Bioses, and RMAd my video card for a new one that had the same errors, amd re-installed windows several times. All parts worked perfectly when tested in another computer. My final fix was to swap my nice ASUS motherboard for my son's cheapo, but newer, Biostar. Everything worked fine with the Biostar, and my old Asus board runs like champ with my son's ATI 9200. For some reason, though, my Asus motherboard and 6600GT would just not work reliably with each other no matter what I tried.