Computer shuts off randomly..HELP!

Ok, i've been dealing with this problem for about a week now and it is driving me crazy. I built this computer about 6 months ago and it has been running fine with no problems whatsoever. Then one day last week I get home and the computer is off. Once turned back on I thought everything was fine. About 10 minutes later it shut off. And by saying shut off I mean it turned itself off like the power had been disconnected…no warning that it was going to shut down. Then when I tried to start it back up I wasn't getting any video. I tried a couple of other video cards and none worked. Thinking I may have a bad motherboard I replaced it (along with a new processor) with one that had just been bought two days before for another computer I was building. Just to be on the safe side I put in a new power supply too. This time everything booted up fine. After awhile though, it shut itself down again. Thinking it may be a heating issue I kept the side of the case open and turned a fan on. I checked out all the temps for my motherboard and cpu and they weren't overheating. Still thinking it might be my video card, I began using the on-board video instead. That did the trick, or so I though. The computer stayed on all night, but after a few more hours (without me even using it once) it shut down yet again. Also, when it shuts off, I have to unplug the power cord for about 10-20 seconds then plug it back in to be able to turn it back on. I have run a memory checker and it says the memory is fine. I've replaced the power supply, motherboard, processor and tried several different video cards and i'm at the end of my ropes. I don't feel it's a heat issue since all temps seem to be ok. I've searched all over the internet and still haven't found a solution. I am going to try using a different power cord since someone suggested it, but would be glad to get help from anyone here. Also it might help to note that the only thing that had changed on my computer before this happened was the installation of the newest version of DirextX. I have however updated all drivers for my motherboard, video card and sound card to make sure it wasn't a driver issue. Can anyone help? I'm running a P4 2.8ghz, 1gb Ram (2 x 512mb sticks), Win XP professional

Hi Digikatz, You appear to have taken several competent steps to isolate the problem, but with no success. Please excuse me if my recommendations sound too elementary. When chasing down a possible hardware problem it is best to remove almost everything. Then boot with the minimum elements. This would include using the onboard video/audio, no peripherals such as CD-ROMs, printer, speakers, etc. and only a single stick of RAM. It might also help to back off to PS2 type keyboard and mouse if you are using wireless or USB types. If you find success with the most basic elements, then add the second stick of RAM, and then one Card or peripheral at a time, making one change in BIOS to activate/deactivate the feature as you progress. Hopefully, this would allow you to find the culprit when adding an item replicates the problem. Since you are building this machine, you will also be considering the motherboard mountings and spacers such that no "short" could be causing the problem. The most basic version of ruling out "short circuit" problems related to mounting screws/etc would be to lay out the machine on a table top outside of the case. I wish you well, and hope that you will post back with your progress.