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,

Download HWmonitor and check the temps, turning off under load sounds like the CPU thermal protection to me. When was the last time you cleaned the heatsinks?

I wouldn't call streaming from the net (like a preview for a tv show) having the PC under "load"

Honestly ... over 12 months ago ... good enough place to start ..
I guess maybe when I opened the PC to switch power supplies I might have moved some dust around

The failing power supply could have caused damage to any other component.

-Clean it out
-Memtest
-Chkdsk the drive
-redo the TIM
-Check mobo caps
-Swap out videocard or test it someplace else

Just want to know, what brand was the old PSU and what brand is this PSU?

as an complete WAG (wild *** guess) I'd bet that the power supply you have now is simply not up to the job, and a circuit protector is being triggered because it's bever overloaded.

That is my inclination as well. If it was CPU Thermaling, you woudn't need to power-cycle the PSU to get it to power back up, correct?

exactly