Help! My PC's FPS Keeps Tanking: A Strange Gaming Mystery

Hey everyone! I’ve got an interesting one for you guys that I can’t seem to find an answer to. So yesterday, completely randomly, my PC started having major FPS issues when playing games. When I boot up a game, all is well; the FPS is as it should be. However, after an undetermined amount of time, whatever game I’m playing TANKS in framerate and stays that way. I tested this foremost with League of Legends. Typically, I can play that game at max settings locked at 120fps(I can go higher but my monitor's refresh rate is only 120, so I cap it). Randomly throughout a match, the game will slow down to 50-60 fps and struggle to maintain that. Now here’s the weird part - if I open Task Manager and look at performance, it doesn’t add up. My PC isn’t even close to using max power on that game AND, even STRANGER, opening task manager FIXES the FPS issue and it goes back to 120fps locked until it does it again, and then I have to open task manager again to fix the framerate. Rinse and repeat until the match is over. It does this same thing for several games and I cannot for the life of me think of what might be causing this or a solution, so I’m open to ideas if any of you have ever encountered this before. Hardware is as follows: - Nvidia 1660ti - Intel i7 10750H (2.6ghz) - 16gb RAM - 512 gb NVMe SSD Have you ever experienced anything like this? I'd appreciate any suggestions or solutions you may have. Let's solve this strange gaming mystery together!