my computer shuts down and then restarts while playing games such as call of duty and gta iv?

so, my new computer shuts off and then restarts while playing high performance games with full Resolution 1920x1080. it shuts on call of duty 2 + 4 and sometimes shuts with gta iv. it shuts down without no warnings or anything, then restarts (like when i turn it on). The cpu is core2quad q9550 motherboard is asus p5q3 ddr3, ram memory 2x2gb ocz ddr3 1600mhz graphic card zotac gtx 260, power supply 600watt thermaltake toughpower(which is clearly enough) and my cpu cooler is thermaltake golden orb ii fan. i checked my ram, its ok. i recently put graphic card cooler which contains 3 fans. my computer is not covered, it has it's window open.

what could that problem be? how can i fix it?

thx in advance please help me

any more things u need hussam_abu_hanna@hotmail.com is my e-mail