Blackscreen while playing games every 10-20 min

Hi, i have a problem that is driving me crazy.

For a few months now i cant play games for more than 10-15 min.

Example, im playing cod 2 multiplayer, battlefield 4, assassins creed and suddenly my monitor goes blackscreen, sound goes sometimes zzzzzz, the keyboard si frozen, i can still hear my friends on skype, teamspeak....
Now i have to turn off my power supply and wait a few seconds then i turn it back on and start the pc, if i just reset my computer it wont start the screen will still be black. I have test everything and nothing seems to be wrong but i dont believe that, something must be wrong. The temperatures are all normal. I ve done clean format a few times. Im running on win 7 x64

My specs:
Intel i7 920 - 2.66 GHZ, 1366
ASRock X58 EXTREME6 LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
Seagate ST31000528AS 1TB hard drive
G.SKILL 2GB DDR3 - 1600, 6 GBPI x 3 RAM
Nvidia GeForce 295 gtx graphic card
Thermaltake Qfan 750 W toughpower power suply

thx

Napisal se post na eni angleški strani in sm dal sam copy paste, upam da ni to problem.

Do you have overclocked CPU or GPU?

Smo na slovenskem forumu.