League Minimising out of Windows Every Game

I updated Windows yesterday and now have an issue playing the game. All works fine until the match is about 3 minutes in (give or take), then the game will randomly minimise and take me back to desktop. This happens every single time. I can click on the League icon (on the toolbar) again to rejoin, but the game effectively throws me out for about ten seconds while it maximises the screen again. Ten seconds idle is plenty of time to die! Anyone else suffering the same? I've tried looking at task schedulers but have nothing enabled that would interrupt.
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