My PC freezes when i get into a game

When i get into a game after about few minutes it freezes or bugslpats for unknown reason. It just freezes and afterwards my screen turns black and its like i turned of pc, it says "no signal", button turn orange and i have to rs pc, I bought completely new pc just a few days ago. I tried contacting riot support but so far nothing. Before I got this pc, on old one, i had similar problem, game would freeze but only on about 15-30 seconds 1-3 times per game .
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