Black screen for a couple of seconds

Hello everyone,

I've already been looking on the internet for any solutions and have tried some but nothing has seemed to work so far.
When playing a game (League of Legends), my screen sometimes randomly turns black and recovers itself after a few seconds. I already deleted previous drivers and installed the latest drivers (even the beta one from nvidia). I've also run Prime95 but that only checks my CPU which seems fine. I've been having this issue for a couple of days now. Mostly, it happens when i'm in game and my temperatures always seem fine. But sometimes it happens when just browsing the internet, after playing League of Legends. I would like to try to solve it without needing to replace any hardware parts because all my parts seem fine from temperature, etc... It would be hard for me to get a part to test my PC with because i don't really know any other people with custom PC's. I have a gtx660 Graphics Card and a 600W GS600 Corsair PSU. (As i've read on other forums, these two parts seem to be the major problems). I've seen many forums and would like to ask direct help from the pro's because i've seen so many things to try that I don't know what to try anymore.

Sorry for the long post and for my bad English. And thanks already for helping me!

edit: If i start the game now, my screen turns black and doesn't recover anymore. I have to hard reset my PC.