Computer simply powers off sometimes - How to diagnose?

I have an old P4 computer that I've been using constantly for about 5 years, about a month ago the Power Supply (500W) started making a high-pitch sound - so a few weeks ago I went to a local store - bought a new one (525W) and put it in ...

Everything worked fine at first (simply browsing the net, etc... basic stuff) until 4 days later we tried to play Unreal Tournement and within the first 30seconds in-game the computer simply powered off - this was reproducible on-demand... This also started occuring sometimes when watching shows (either AVI or streamed on the web)...

The only way to get the computer to power on again is to flip the switch in the back (on the Power Supply), wait 30-45 seconds, switch it back, and then power on the PC... Then everything works fine until the next time.

Obvisouly my first reaction was "defective power supply", so I returned it and replaced it with another one (same model) - first thing I did was test playing Unreal Tournement and movies and it worked fine for the first 3-4 days ... now yesturday while streaming sosme TV shows it happened again ...


That's pretty much the entire story ... my question ... how do I troubleshoot the issue?
- Is this a video card issue (only occurs when playing games or watching movies)?
- Is it a power supply issue (too strong? not compatible? sucks because no more return)
... RAM ... Mobo ...? did I just get unlucky?

Is there something I can install, some kind of monitoring software? Something that can help indicate where the issue lies ... Given that this is intermittent I can't just start removing hardware piece-by-piece...

Any advice would be much appreciated...
Thanks,

So you have to flip the back switch off to get it to restart? Not sure if it applies in this case, but the only time I remember having to do that was to reset the overcurrent protection in the PSU since there was a short in my system. There may be another bad component (or you are just really unlucky).

For the hundredth time today don't use cheap power supplies!!! Buy a corsair or OCZ and there are some other good brands aswell like coolermaster.