Originally Posted by angelkiller
Not to discredit what you said, but I'm not totally convinced it's the PSU and here's why. I don't see how the PSU could be bad if it can pass stress tests and 20min of a game.
Routine is for a defective supply to boot a computer. Then (intermittently) cause a system crash. Nothing you have done will give a definitive answer.
How to know - without any doubt - whether the power system is good or bad. Power 'system' because the supply is only one component of that 'system'.
First multitask so that all peripherals are being accessed. For example, the game would execute complex graphics. But also searth the hard drive, while downloading from the internet, while reading a CD-Rom, while powering something via a USB port, while ... only then are you truly stressing the relevant parts. Every component must do something that consumes power.
Locate cables from power supply to motherboard. With a digital multimeter on 20 VDC, measure any one purple, red, orange, yellow, green and gray wires. Post those numbers (to three digits) here to learn about things you did not even know were part of the power 'system'.
Only way (short of $thousands in test equipment) to identify a power supply as good or bad requires those numbers. This is your only useful solution to learn if the power 'system' is good or has a defect.