Having to reinstall League every time i want to play (FPS problems)

First game, I have 150-200 FPS. In a couple of games, it drops to ~100 FPS. Then, eventually, after 5-10 games, my FPS is 15-30, which is absolutely unplayable. My PC is OK. The only way to fix it seems to be reinstalling League, then it goes back to 150-200 FPS. I tried Initiate Full Repair, and it doesn't work. I also tried delete Logs, delete Config, also doesn't work. Drivers are up to date, no programs run in the background, etc. Antivirus software deleted, no viruses, etc. Please tell me what game files i need to change/delete, instead of reinstalling the game every time.