If you have the spare parts, I would definitely recommend swapping out the power supply for a known good one. The motherboard could also be the issue. If you want to do it yourself, make sure you have an anti-static wrist strap to avoid shocking yourself or the system. First try unplugging everything except for the cpu/heatsink, ram, and motherboard. No DVD drive, no hard drive, not even the cables for the usb or audio ports. Try it again. If it still doesn't work, replace the power supply. I had this problem, and it actually turned out to be a flap of metal on the case itself, that was making contact with the motherboard. Pull the motherboard out, and visually inspect your case. If none of this works, return the motherboard for a replacement.