This problem could be many things so trial and error is the key here. Could be shot hardware like the mobo most commonly or the PSU being overworked. Listen to your fans to see if the ambient noise they make drops off and on. Could also be major software conflicts in which id just do a low level format and reinstall to also push out bugs. Also go into your bios to check the voltage settings and temperatures of the cpu and system chipset. Also use a ram tester software, which you can get for free from download.com, to check for errors. Could also reflash your bios to the latest version for the hell of it. Could also try your parts out on other machines to see if you can duplicate the error and isolate the problem hardware. Hope this helps..
F-DISK-Format-Reinstal DO DA!! DO DA!!