Just go old school troubleshooting and start exchanging parts with another PC. Last thing to exchange would be the motherboard. EDIT: looks like you did that with some parts already. I try to discard the possibility of saying "such and such part is new!" because even new parts can fail. Now, even if you install new parts and drivers are good, you can still have some fail. This seems odd, nonetheless. Is the PC going into sleep mode, etc.? Can you list what you've exchanged already? Cable, video card, power supply?
