Okay so to run you the short story my pc broke about a week ago and it was an error with windows but I didn't realise that to start with.
So to try and fix at this point and problem I had no clue with I tried factory resetting my pc and restoring it but neither would work. So i decided to open up my computer and just get rid of the dust in my pc ( not to much in there ) either way I took of the fans and the CPU and clean of the ''dust'' turned out it wasn't dust it was thermal paste. As i fixed the problem ( had to re do windows 10 then factory reset it ) my PC looked like it was running fine. I play a game called league of legends im sure you would of heard of it and its games can last up to around an hour but generally around 30-45 mins.
I get 150fps which is fine its great but after about 20-25 minutes of the game I drop to 50fps for no reason that I can see. I've checked temp and it seems around 40 so I have no Idea what the problem could be ? it does it every game tried re installing drivers Iv'e ordered some more paste to apply when it arrives but I don't think its going to fix my problem ?