Grey black screen :(

[Problemnot]

Hello
So while playing LoL i usually get a blue/ green/ black/ grey screen when am in game and my computer just freezes in order to get back in the game i got to manualy restart the computer (press power button).
Am using Win7 64bits Quad core Q6600 , gtx 260 and 4gb of ram
PS : i full stressed the gpu with fumark and 3dmark noproblemo

 

[ffg7]

do you have another videocard or can borrow 1 for testing purposes to see if still has an issue. what is the total wattage of your psu?

 

[Problemnot]

I have a b-move 600w