Low FPS and GPU performance in menu

Sorry if this has already been posted. I couldn’t find this specific issue.

Over the last two weeks I’ve seen an extreme decrease in performance. Yet, it’s incosistent. My average fps is around 70 to 80 maybe down to 50 in a large city, which is more amazing.
The problem arises randomly after a few minutes in the game, the fps drops and fluctuates between 0 and 5 and gpu usage % around 0 to 20. What I find odd is that this happens in the menu also, not just while flying. I would assume the menu is less demanding so it should not happen in there but it does. I have the latest drivers installed.

Any suggestions?