This was always a problem since building my rig in Feb 2013. It would only really shut down after long hours of max settings. Now when im running lowest settings its just shutting down. I've already replaced the case, applied new thermal paste to the cpu and cleaned EVERYTHING top to bottom as well as replacing my old 450w psu and upgrading to a 600w psu. Ive also reinstalled windows to a fresh brand new harddrive.
Temps on my card when gaming never get over 70 Celcius and my CPU Never tops the low 40s.
Case: HAF 912
Case Fans: (x6) Cooler Master 120mm Silent Blue LED
Mainboard: MSI 760GM P23(FX)
Processor: AMD Athlon 2 270 Regor @ 3.4ghz
GPU: HIS Radeon HD 6750 1 GB
PSU: Corsair 600w Builder Series
RAM: (x2) Corsair XMS3 4GB 1333MHz DDR3
Hard Disk: Western Digital Black WD1002FAEX [1 TB]
Monitor: Asus p186h 19'' LCD
OS: Windows 7 Ultimate SP1 (64)
Could be a connection to the mobo that cuts the power. That's the only thing I can think of that you didn't replace. On the bright side, you can get a really good/cheap mobo combo for a newer socket type.
that what i was thinking originally cause it has been doing this since i built it. just very very rarely. now its doing it 3-4 times a day when i try playing games. I was told that it could very well be my tempature monitor on this mobo since its very small and the gpu runs hot that maybe the mobo was picking up the temp from the gpu.