My computer will switch on, stay on for about 3 seconds, and switch off. It just keeps doing this over and over and over. I have removed everything and replaced the power supply. Right now I have the motherboard (Asus P5B Deluxe w/wifi) out on my bench with power supplied. All expansion cards are removed and the RAM is removed. All I keep getting is on, off, on, off. If I remove the 12V eatx supply the unit will continue running but nothing more happens. It won't finish the test or anything. Forget about actually booting the OS. My thought is that somethinig is shorted on the motherboard - possibly from over heating. It did do something wierd that caused me to shut down to begin with. My monitor "fritzed." it got really light and had lines in the display. I thought that I had gotten a power surge or something. I restarted and got the same display. I shut down and let the unit sit overnight and the next morning this started. Before I declare the motherboard dead I thought I would throw this out for comment. thanx

Looks like a sticking power switch. Power switch is a momentary contact switch and if it stays depressed, PC will shut off after about 3 to 5 seconds.

Disconnect power switch from motherboard and use a jumper to short power pins. Remove as soon as CPU fan starts. You can borrow a jumper from an optical drive after disconnecting drive.

Using a small flat blade screwdriver to short the pins will also work. Exercise caution if you use this method.

commented: Good info +2