Weird FPS drops

I have this problem where the game would start fine with 50+ fps but after not even 2 minutes the fps start to drop.. they get down to 28 or even 16 fps. im running the game on my laptop with high performance mode, with the charger plugged in, so removing the charger raises the fps back to 40 50+, and plugging it back in also raises the fps, but in both cases, after about a minute the fps drops down again. anything i can do to stop the fps drops??
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