Game Crash

Whenever I play League of Legends and im in the game, almost every game my monitor goes black/red/green/yellow with some vertical lines. I hear the game and my computer is working,but i can't do anything so i have to turn off my computer. Restart takes for good 5-8 minutes and that almost every time gives me low priority queue-leave buster... It's really frustrating and annoying , the thing is it ONLY happens when i play League of Legends !
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