Black Screening Problem

Whenever I'm on the LoL client or in game, it sometimes causes my screen to turn black for a few seconds and then it comes back on. My graphics card is up to date and I've tried fixing the client and lowering my graphics settings but nothing seems to work. I'm running on Windows 7 and everything is up to date. It doesn't happen with any other game and it doesn't cause my secondary screen to black out either. Please help!
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