PC shuts off while playing Mafia II and Arma, why?

sacred afro

New member
Here's my specs:
650w antec PSU
Apevia X-cruiser 2 case
gigabyte GA-P55-usb3 motherboard
4gb ddr3 RAM
intel core i5 760- 2.8 Ghz
BioStar ATI Radeon hd 5670

When i was playing the mafia 2 demo and the arma demo, it would lag a very small amount, then the pc would shut off entirely. I meet all minimum requirements, and almost meet recommended (3Ghz CPU, i have 2.8)
While building, the CPU was removed multiple times for hardware issues, but now is fine. Im wondering if it is the heat grease for the cpu? it could have got smeared off.
Please help i really want the mafia 2 game but im not buying it if i cant run it!
thanks for the input.
 
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