I had similar issues with faulty RAM installed.
Download
this and burn it on a CDRom (Yes, the download is only 39kB).
If you are using Linux too or have at least an installation disk lying around somewhere,they usually offer MemTest86 in their boot menu too.
The tool's test will run continuously, so don't be surprised it it never finishes.

Give it one or two passes. If it discovers more than 20 to 30 bit errors per pass, the RAM is a safe bet. In this case, remove all but one RAM module and re-run the test to discover which module is failing.
If memtest86 doesn't reveal any errors, I'd also blame the motherboard. I don't believe it's e.g. heat problems since newer nVidia cards automatically reduce the clock speed once the GPU temperature exceeds 127 celsius (driver setting, if you check the nVidia driver settings, there's an audible alert somewhere that you can turn on).