Power supply / mobo issue

[SCRawl]

I've been working on a Dell Dimension 8300 (desktop) that belongs to my daughter's school, with the following complaint: it shuts itself off on occasion, without warning, and won't turn back on for a while. The little power button in front of the case blinks a faint yellow after it powers down.

I looked up the problem, and it appears that many, many people had a problem with their Dells of that vintage, and the problems were almost always with the power supply. So I went to swap it out with a new one I happened to have laying around and, no, Dell uses fucking proprietary MB connectors with their power supplies. Fucking Dell.

So, I ordered a new one from Dell. $75 including taxes and shipping. Fucking Dell. I put it in last night, did some maintenance, and left it on for the night. This morning: it powered itself down. I turned it on again and went away to see to other tasks, and when I came back a few hours later, it was off again. Same problem, it wouldn't turn on, even after de-plugging it.

My fear is that the problem is with the motherboard, but I was hoping that there would be someone here who could point out another possibility before I break it to the school that (a) the computer is toast, and (b) I just threw away $75 on a redundant power supply.

Any thoughts?

[Edi]

Remove the processor and heat sink and see if the paste there is correctly applied. I've run into a case where some moron actually left the paper covering on the paste and just plonked the heat sink down there, adding an extra layer of insulation. After that was removed, no more overheating problems, which caused the random shutdowns.

Otherwise it's a mobo issue and you might as well toss the whole fucking piece of shit and buy a newer, slef-built one from cheaper end components, especially if you can use the RAM and HD from the old one.

[Uraniun235]

Check the capacitors on the motherboard for bulging and/or leaking.

[SCRawl]

Remove the processor and heat sink and see if the paste there is correctly applied. I've run into a case where some moron actually left the paper covering on the paste and just plonked the heat sink down there, adding an extra layer of insulation. After that was removed, no more overheating problems, which caused the random shutdowns.

Otherwise it's a mobo issue and you might as well toss the whole fucking piece of shit and buy a newer, slef-built one from cheaper end components, especially if you can use the RAM and HD from the old one.

You know, I hate to admit it -- I always hate to look stupid -- but I think that I can chalk this one up to human error. Okay, sub-human error.

It appears as though I took out the old power supply, set it down, and then put it back in the computer. With the new one sitting right next to it.

Thanks for the tip, in case the PS turns out to still be a dud.

[SCRawl]

Check the capacitors on the motherboard for bulging and/or leaking.

That was actually the last thing I did before sinking dough in a new power supply. But as my previous post indicates, I'm not nearly as clever as I thought I was (or should be).