I'm having a problem with my desktop computer booting. It's sporadic, and I would appreciate your thoughts on what the trouble might be. When I first turn on the computer it will power on. I see all fans running and the HD light is on. After about 5 seconds it shuts off. After repeating this several times it will stay on and the HD light will stay on, but no POST beep. A reset does nothing, only holding the power button will turn it off. After one or two more quick shut offs, it will boot and I get the single POST beep and the computer will work fine. It won't shut itself off, it works fine for as long as I want to use it. Could it be the power button ? I don't think it's the power supply, it works fine after POST. Maybe there's a problem with the power good signal. How would I check that? Thanks for any input you might have.
Athlon XP 2800+
Asus A7N8X mobo
1 GB Corsair RAM
WD 80 GB HD
Liteon CDROM
LG DVD Burner
ATI Radeon 9800 Pro
No problems in Device Manager, all device drivers are up to date

Hi welcome to the forum it could be a power supply problem buy the sound of it but could also be a hard drive problem "HD light will stay on" this wouild indicate that. First I'd download data life gaurd diagnisic form here Downloads and test the hard drive drive. I will see if I can get a voltage meter for your mainboard

I ran the HD test and it passed. SMART was OK also. I'm gonna research testing the power good signal. When I have time I'll strip the system down to RAM and HD but I'm not confident it's in the optical drives or Graphics. I ran MemTest and it found no errors. I hate to buy a new PSU to find that's not the problem. All the capacitors on the mobo look fine too. Thanks for the effort and if you have any other ideas lemme know!

If the hard drive diag checks out and the caps look ok I'd say it would be the power supply for sure it looks like you don't have much of a choice you will have to try one it's cheaper than a mainboard.lol When it won't boot do you get any beep codes (beeping noises) this will indicate what is wrong?

No, I get no POST beep when it shuts down quick or when it stays on and I just see the HD access light stay on. That's how I know it's not going to boot properly. I'll recheck the PSU fan and see if it's not starting up when I get the quick shut off. It's a good PSU - PC Power and Cooling but that doesn't mean it can't go bad.

If the HDD light stays on when you have the problem it would indicate that it has a problem but it could also be the mainboard. If that red light stays on it will be either the hard disk controller on the mainboard or the hard drive is faulty. I would backup all important data. When the red light stays on can you hear the HDD spinning?

I hear the HD spinning. It's like it's trying to access the MBR but it will not POST. One other thing I neglected to mention is the first press of the power button sometimes does nothing - no power light and no HD activity light. Second press immediately after power and HD lights up. And if it shuts down after 5 seconds, a press of the power button (after waiting 10 seconds) lights the lights very briefly, like the power drained. Just another piece of the puzzle.