I'm after some differential diagnosis with my PC.
Issue is as follows:
My processor is an i7 3820, overclocked to 4.4ghz at 1.3v; which has been a perfectly stable overclock for the past 8 months or so. Just recently my computer has been hard crashing after about 15-20 minutes of playing games (happens with all games). For the most part there is no actual BSOD, just freezes and needs to be reset.
I immediately thought that this may be a CPU issue so i restored the default bios settings, but the same crash still occurs. Following this I ran 80 cycles of intel burn test at both stock clocks and at 4.4ghz, both completed with no issues at all (core temperatures reached 65 degrees celcius @4.4ghz, which is marginally below tmaxjunction listed by intel, the cpu temperature sensor reached about 55).
Next I thought possibly GPU since it's been overclocked from 850mhz to 1175mhz for quite some time (again, no issues at all until now, I have set an aggressive fan profile which prevents the GPU temperature from exceeding 70 degrees.) I ran a few full 3dmark test at both stock and overclocked speeds, and various other GPU benchmarking tools, none of which caused the hard crash to occur.
 Forgot to mention my GPU is an Asus 7850.
 my PSU is a corsair HX750; all system voltages remain well within acceptable ranges during load and during idle.
My next thought is the RAM but before i run memetest overnight I was wondering if there was anything else which might be worth considering. Thanks!