My game has started to randomly tab me out in the beginning of the game

Usually happens early game anywhere from the start to early laning phase. Forces a tab out and I have to click back into the game. Pretty frustrating especially if it ends up getting me killed. So what the %%%% is this and how do I fix this? I play in fullscreen. Any other option gives me bad FPS so preferably a fix that still allows me to play the game in fullscreen.
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