FPS drops after a certain time

Ever since a week or two ago, I would hit a certain time and my grams would eat it. I would sit on 60-72 frames all game before. Now I hit like 10-15 minutes and now I’m at 4-25ish frames. My frames are like normal before that. I don’t have anything running in the background except for iObit uninstaller and malware fighter. I will post more specific specs later, but my CPU is an Intel i7 @2.67GHz with a Nvidia Quadro FX 880M. My gpu is old but I heard that league is more taxing on your cpu. What should I do?