Computer hard crashes when playing games at specific points.

There is a specific point in every game i tried (the one I've found the most consistent is Far Cry 3) where the computer completely cuts power and has to be unplugged and plugged back in to turn back on. I've tried replacing every part i could with other parts and have looked everywhere for a solution but i cannot find any. I've tested the hard drive and the RAM and the G-card and the problem consists despite using different of each (different Hdrive Gcard etc problem still persists). Im in the process of testing the PSU but I don't have a replacement on hand. The problem occurs with a fresh install of windows 7 ultimate on the original and an empty hard drive i had laying around.

the only thing about this rig that is unusual is that the PSU did not have an 8 pin connector for the motherboard, so i bought an adapter that turned two of the auxilliary power connectors into another 4pin and used that. I don't think that that would cause this problem as everything successfully POSTS with this adapter set up and if the adapter didn't work then the computer would not turn on at all. i don't have another PSU to test.

originally the RAM were GSKILL 2X4GB 2133 and thought the problem was bad RAM timing since the motherboard only supports 1333 so I replaced the 2133 with current 1333 but the problem was still there.

specs
windows 7 ultimate X86
AMD FX 8150 X8 processor
NVIDIA GeForce GTX 550 TI
TURBOLINK ATX 500W PSU
GSKILL 2X4GB 1333 DDR3 RAM
ASUS 970 Extreme 3 motherboard