Low fps and it's really frustraiting please check this post out.

I bought this pc two years ago, it worked fine with LoL until recent year. My pc runs on 120 fps solid when i got it. It does even flinch if i have medium low graphics and capped in 120. Pc runs in Geforce Gtx 960 with amd 6300 six core processor. I experience huge fps problems in league. I start the game with 120 fps and it drops to 100-110. I get to lane with my minions the fps is 90, when enemy minions come in view it drops to 70. After playing for 20 minutes max fps i get is around 60-70. everytime after 20 minutes i get to lane fps goes to max 63. Most of the time i run with 50-60fps. (its not capped) in teamfights it gets to 30-50. I've tried everything, reinstall league, change graphics to defaults, try playing with nvidia settings, reinstalled windows, tried hextech repair tool, high or low graphics the fps is same, graphic cards are upto date, everything is normal. all other games works fine without any issue, the problems is just in league. When my girlfriend plays on my pc she has 80fps solid, My account in other hand goes from 120 to 50 in 20 minutes and stays there 70% of the match.