It may not necesarilly be a case of the PSU going on the way out - if your PC actually physically shuts down (and more specifically - won't turn back on), that's also a good sign that your processor is overheating. The best way to diagnose this is to download something like CpuCooL (do a quick Google search for it, I can never remember the URL). The warning signs you're looking for are if your CPU is over about 72/75C if it's a P4 and... about 90C? for Athlons. Either way, have this open and a windowed copy of SRB2 or some other game/program that's CPU intensive and see what temperature it gets to. If it's going above those figures, then here's the solution:
- Crack open the side of your PC case and have a look at the CPU fan - if it's actually stopped moving or is moving slower, this is your problem.
- Sometimes, this is due to a thick layer of dust or a stray power cable getting caught in the CPU. 3 minutes with a tissue should get some of the worst of it off.
- If that fails, or if there's nothing that looks like it could be concievably obstructing it, then you'll need a new heatsink. I've heard good things about Arctic Cooling heatsinks, so this may only be a case of paying $25 to fix it.
- If the new heatsink fails (and you've seated it / applied thermal paste correctly), it may well be your CPU chip that's buggered. With any luck, it won't come to this. ;)
Of course, your PSU may still be to blame, but the only way you'll find out is if run CPUCooL - if the temperatures are fine, your PSU is probably knackered. Does this happen in other games/intensive programs? Keep us posted.
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Johnny