Continual restarts (no POST)

[NoneMoreBlack]

Hi, I've built quite a few computers and troubleshooted a good amount of hardware issues, but this one is new to me. When I power on my computer, I get a burst of life from the GPU fan, the case fans, and the power-on LED, which then all simultaneously cut out about 2 seconds later. The odd part is, then it will continue to restart and die at about 3 second intervals until I cut the PSU switch off, except except for one last click even after the PSU switch is off. No POST, no beep codes, nothing. At first, I thought the power switch plugs simply got loosened somehow, as it seemed as if the power switch was continually shorting.

The conditions leading up to this are thus: I just took off the stock CPU cooler and threw on a Tuniq 120 tower. Now, as I said I've done this stuff plenty of times so I don't THINK I screwed anything up in doing this (the install required pulling the whole mobo out and attacking a bracket to the underside).

Additionally, just after installing it, the comp started up just fine, and I ran it for about 10 minutes with no problems. I ran a variety of programs (SpeedFan, CPU-Z, ATI Omega control panel) to verify that the CPU was running fine and not overheating; a steady 30C. Then, realizing I had forgotten to reconnect my front side USB and sound, I turned it off, and immediately this restarting problem began.

The measures I have taken in attempting to diagnose this:

Waiting, trying again
Checking connections, particularly power switch
Pulling all components down to the CPU and replacing them one at a time
Disconnecting mobo power, reconnecting, etc
Resetting CMOS

All to no effect. I am terrified that I somehow burned out the CPU or mobo, but I don't see how that could be the case since it started and everything was just fine, especially CPU temps, and it didn't cut out forcibly; I shut it off myself. My guess/hope is that perhaps the PSU cut out, and the timing is just a coincidence? This isn't at all like dead PSU's I've had in the past, and does smack to me of a dead mobo.

My components are as follows (computer is just over 6 months old):

Gigabyte 965P-DS3
E6400 Conroe
2 GB Corsair DDR800
ATI Radeon x1900
Thermaltake TR-420W

Thanks in advance for your help.

[NoneMoreBlack]

New (and shocking!) development:

Forgot to try this combination of plugged/unplugged devices before. When I unplug the ATX-12V power supply to the CPU while leaving the 24-pin ATX mobo power in, it starts up as normally as it should with no CPU; at least it doesn't do the continual restarts. However, plugging that 12V ATX back in brings me back to the restarting problem.

So: dead CPU? I cringe to think so, as that's basically the most expensive possible option, but it seems the most likely. Anything else you would try?

[mechbas]

You didn't note that you actually CHANGED the PSU.

[NoneMoreBlack]

Changed the PSU? I don't follow. I haven't done anything to the PSU.

[mechbas]

PSU power supply unit.
you noted that you disconnected it, you didn't note that you actually tried another. in the day when plastic standoffs are used to mount the board it might be a good idea to find out if the board is actually grounded to the case. in the day when x486 was king it made the difference. cost me a trip clear across town to find that out.

[Chromium]

The bad part is this, i've had something VERY similar happen to me, may not be the same as yours but mine (believe it or not) was the keyboard. Dunno what happened to it but anytime i turned the pc on it would die, took forever to realize it was a KB problem. Yours does sound like a PSU though, maybe a bad capacitor in there, coincidence or not if you can find a replacement temporarily i'd try that before chalking it up as a loss of the mobo. good luck.