Computer turns off without warning in games

[iibenii]

I recently built my own pc and everything was working.
i put 3d mark on to test my system and in the 2nd test it just shut off.

i thought is was the graphics card so i replaced it and ran 3d mark again and it worked.
later that day i tried to play minecraft and it shut off again without warning.
i thought the psu might be at fault (i have a powercool 750 w)
but i removed the gpu and ran it on my g41 chipset and it shut off again.
could the cpu be at fault i get about 40 degrees on idle and 50 in minecraft so i dont know whats wrong.
my specs are:
asus P5G14T-MLX MOTHERBOARD
2gb crucial ddr3 ram
intel core 2 quad q6600 that i got used
BFG Gforce GTX 285 OX
Power cool 750 w psu
Windows 7 ultimate 64 bit

[Dogg]

Shutdowns are almost always a hardware fault. Typically, that would be motherboard or power supply.

However, you stated "but i removed the gpu and ran it on my g41 chipset and it shut off again." Which would seem to indicate the gpu is faulty.

[iibenii]

I thought the gpu was the problem but it was replaced with another one and the same thing happens. it doesnt matter whether the graphics card is in or not so its not the gpu and it cant be the cpu because i ran prime95 for 24 hours with no shutdowns. Just to add when plug the psu in and flick the switch a anoying high pitched whining sound comes on and goes when i press the power button.
could this suggest that the psu is faulty?

[gcavan]

Ringing or whistling or any noises coming from your power supply other than that produced by the fan moving air is a sure sign it is failed/failing. Confirm the sound is being produced by the supply and replace it. In any case, the Powercool would have been my first suspect.