Computer restarts when playing games

Computer just switches off and then reboots itself. when playing DIABLO 3, BF4 and sometimes even League of legends.

I have up to date drivers and no viruses. heat from graphic card and cpu is fine.

I have windows 7 64bit,
i5 750 2.6ghz
4gb ram
ace psu 550w 12cm red fan (I think is the problem)
hd radeon 7800 series 1gb
motherboard is intel DP55WB

I think its my power supply. I have looked it up on amazon and people have had similar problems.
prefer some other opinions if possible thanks
 
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