FPS drop in the middle of the game

I've had this issue with FPS for the last 4/5 months, i thought it would fix itself with any update, but turns out it didn't.

When I get in a game (any gamemode) my fps float around 300/400 fps, after some rounds, the FPS drop suddenly to 110 or lower. I've checked and it's not an overheating problem, because temp doesn't get higher then 40ºC. If I restart the game, it fixes the problem but after some rounds it just does the same. It's very annoying because I can't constantlly quit game and reconnect. If any kind soul had the same issues and got them fixed I would love to get some help.

Please