Problem fps stability

I have a big stability problem in my games. At first everything runs normally (I get a stable framerate like 80 FPS on KDC2 on very high settings in ultrawide), but after about 5 minutes I get a 20-FPS drop and my minimums go down to 15. It’s just unplayable. I thought it was due to overclocking, so I reset everything to default. I relaunched the game but nothing changed.

Then I thought it might be caused by Wallpaper Engine and animated backgrounds, but the option is disabled and doesn’t run when a fullscreen app is active. Just to be sure I closed the application, but nothing changed. I checked my task manager: my processor was at 30% and my memory at 40%. I don’t know what’s causing this…

32gb ram Ryzen 9800x3d Rx 9700xt.