League lagging (fps)

Hi guys! The first game I play (after reinstall League) is fine, I have usual 200-300fps, during the game, it drops to 100. The second game I already only have 60-80 fps and in the 3rd game 20-40 (what is unplayable) after the games, I restart my PC and it still doesnt work, for 1 weak I reinstalled League every single day to play 2 games a day (downloads require long with my Internet). I backuped my PC, set windows up again, but everything wont work for me, as you already noticed, I have a good PC, not MONSTER PC but it should be finde, I can run Overwatch, max. Settings. Thanks and bye!