Slowing Down CPU

Lately, whenever I've been playing Minecraft, after 5 or 10 minutes the CPU slows down to under 0.5 Ghz and the fans stop running (and obviously the game lags uncontrollably and I have to quit it). It used to happen, but I got TPFancontrol and the problems stopped. However, when I replaced my thermal paste, the problem started occuring again. The strange thing is, it only seems to happen on Minecraft (it doesn't happen on SolidWorks, and I'm pretty sure it doesn't happen on Valorant, but I can't be sure about that one), and when I went to take apart my laptop yesterday, I decided to play Minecraft without the back housing on, and it was just fine. I am very confused, because the problem only started after installing new thermal paste, but the problem also seems to be related to the housing, and not getting enough circulation. I'd like to think I know what I'm talking about, but I'm stumped. I have no idea what's going on.

I have a Lenovo Thinkpad P43s with an i7-8565u, 24gb ram, and Quadro P520, and when it runs minecraft, it runs it just fine at around 110 fps with Optifine 1.8.9. 

This happens on all versions for me

Another thing: Whenever I unplug the laptop it doesn't happen. It runs just fine.

I also can't disable my IGPU (someone on reddit says that was the problem. It isn't)

Edit: It doesn't happen in Valorant, so it seems like a combination between mysterious thermal problems and Minecraft problems