Weird fps drop at 55 mins +

Hi guys , i think league of legends have a weird bug , the situation is that my new computer handle perfectly league at max setting 1080p on 60 fps cap with v-sync.

I played games where it lasted like 45 mins , no lag still cap 60 fps even during big fight with 10 ultimates. My cpu usage is like 25-30% during league , my computer is super cold and my gpu is a gtx 970.

Then , at 55 mins + the v-sync go to 66 fps ?!! wtf and im getting freeze , then drop from 49-55 ( even outside fights) BUT if i die and i still watch a big team fight im going to cap at 60 fps without any problem.

I tried to remove v-sync at 55 mins + , the fps goes back to 200 and then drop to 49-55 and still do the lag.

sorry for my bad english , i think it could be league of legends the problem and not my computer since im not getting any fps drop from a long period of gaming in any other game way more demanding than league.