Computer Shuts down unexpectedly, then won't restart

Symptoms: The computer shuts down unexpectedly after 15-30 min. of operation, then won't restart until some time has passed - it had shut down on its own, was left until the next day when it booted fine and ran a bit again, but then, once again, it shut down unexpectedly and would not start (the Power Supply fan turns on as well as the CPU fan).
1) I booted into the BIOS and monitored the CPU temp. It’s a P4 1.8 and runs at an idle temp of 42Cº.
2) I booted into Safe Mode and let it run … until it shut down. Then I proceeded to disconnect each internal component, one at a time, while trying to restart after their removal: 1st I disconnected the DVD-ROM, and then I tried to restart, then the RW, then the HD, Floppy, PCI cards, graphic card, RAM, all without success (excepting the fact that the Power Supply fan and the CPU fan run as I mentioned above).
4) I connected another power supply, and the computer started right up. Problem solved!!! Not so fast. Out of curiosity I tried the original power supply again, and it started it right up too! So, that was a little weird, but probably still a problem with the old Power Supply I thought.
5) I did some more extensive testing with the replacement Power Supply (the new one), and couldn't replicate the original problem; things were looking good.
6) I changed the physical location in order to do longer term testing, and upon nothing more than physically connecting the monitor, keyboard, mouse, and power cord, the computer once again WOULD NOT START UP. Now that was really weird, so weird that I checked to see if the power cord was unplugged. It wasn't.
7) I took the new power supply out and tried it in another computer. It wouldn’t start that computer either. I checked the new PS with a tester, and it continues to test good, but now it doesn't seem to have the Amps to get any computer going.
8) Back to the computer itself, now with no PS since I had taken the new one out to test it on that other machine, and, again, out of curiosity, I tried the ***OLD*** PS, and the computer booted fine. Incredible. A little dizzying. So, why not give it a try with the new PS that wouldn't boot either machine? Would you believe it? Don’t - things are weird here, but we're not talking about miracles. The new PS is fried.

It could be coincidence that the new power supply just happened to fail in this circumstance, but I'm doing some reading and searching before I put another new PS on the front line. I haven't dismounted the CPU yet, but it looks fine. It runs warm but not hot. The next step is to test the CPU, right? Nevertheless, I don't think it's the CPU. This looks more like a short problem of some sort? A short due to the case and/or motherboard?

The truth of the matter is that I’m not really sure if I have really narrowed things down to the CPU, motherboard, or the case. I'm not sure that I have really eliminated the other components because everything was reconnected when it failed again, mortally wounding the new PS. It could be any one of the components that fritzes out the PS (in one case simply disabling it for a spell, and in the other complete killing it). I've seen similar things before, but normally everything is reset either disconnecting the external and/or internal power supplies. But this one has me stumped.

Any insight? Advice? Similar experiences? Am I looking at weeks of testing? With the possibility of more PS casualties?
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burrcmConnect With a Mentor Commented:
Two power supplies. One will come back after an overload event, the other will not. The one which will is a better quality unit. The other is a cheapie which has just died. Typically a good unit will start again after power is removed then replaced. But the issue remains. This will be a component - hard drive, CD drive or video card if you are lucky, more likely the motherboard. A short circuit is possible - try the system out of the case to eliminate this possibility.

Chris B
divdoveConnect With a Mentor Commented:
I can't see the psu going capoot with the internals of the computer. I have seen this before and it sounds as if something is over heating. I had same problem once and it did turn out to be the psu fan. When I checked it, it was turning. it turned out it wasn't turning fast enough. It was a bit clogged with dust and stuff. I changed the psu and it worked fine there after. But you have tryed changing the psu and to no avail.

It does sound like over heating to me, I suggest making sure that the new psu is connected correctly before dimissing it as broken. If still not going check all other fans are turning with good speed, if they are then try leaving your case open to see if your pc lasts a bit longer before it conks out. If it lasts a bit longer then it could be over heating somewhere.
manicsquirrelConnect With a Mentor Commented:
Check out and compare their descriptions with your motherboard.  At least once a week we get a computer with the symptoms you describe.  The caps go out in the motherboard, cause a uncontrolled spike in current and shut down the pc.  Eventually this abuse will blow any power supply.
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nobusConnect With a Mentor Commented:
first, were both power supplies the same wattage?
your pc can be underpowered, check here what you need :            
rhutzelConnect With a Mentor Commented:
A goofer but over looked thougth would be:  Have you been using the same electric outlet and or surge protecter/battery backup etc.  It sound funny, but dirty electricity (spikes, surges, etc.) could be your culprit.

HighRevAuthor Commented:
Today I revisited the overheating possibility. I let the computer run until it shut down (about 40 minutes). The CPU never got hot, neither did the motherboard, nor the video card. Everything was also quite cool to the touch.

Another piece of information that I accidentally left out of my original post: the dead power source no longer even powers the fans, and a low level buzz can be heard with the power cord connected. It does test ok, i.e. the voltages are correct, which leads me to believe that it has lost the amperage to be able to handle the load required (I’m not an expert with electronics, but I think this is a correct diagnosis).

A visual inspection of the capacitors checks out good. They are all at good 90º angles, show no bloating, and there is no sign of electrolyte oozing at.

The original power supply is 250W and the failed power supply 420W. The eXtreme Power Supply Calculator v2.0 calculates 195W.

The computer has manifested these same symptoms on three different power outlets, one with a UPS, and another with a surge protector.

I now have the motherboard, CPU, RAM and video card on the bench ready to power up when it wants to. ;-) After failing this morning, it hasn’t been willing to come back to life. Tomorrow we’ll give it another go.
i would suppose a motherboard problem as cause; if the PS on signal stays low, you cannot power on. it should be around  +5 V
more info :
...a,d also disconnect the reset switch, it may be shorting.
HighRevAuthor Commented:
Everything worked fine out of the case. Then I assembled the system in another case, and everything continued working perfectly.

The problem was a short circuit in the original case.

With this kind of problem, the solution is effectively arrived at by eliminating possibilities. All your comments were extremely helpful with that task. I would like to recommend splitting the points with burrcm getting the Answer and everyone else getting an Assisted Answer.

Thanks to everyone for your help!
Glad it is working, however the closing hasn't worked as you intended. Click Help at the top of the page, then look at Closing Questions or can I split the points. If you want to split, you will need to post a zero point question in Community Support requesting that this question be reopened so you can do so. Have a look at other posts in Community Support - you will see how it is done.

Chris B
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