FPS are random

When I play MSFS, I usually fly IFR. However I encountered a problem. Sometimes I get different Performance even tough I am with the same plane at the same airport with the same settings. And I dont mean like 5 FPS differences, we are talking about sometimes 140 FPS with everything green to 80 FPS with everything red and limitations everywhere.

Besides that I also have a second question. Is it normal, that the FPS are getting worse when you cahnge a setting and change it back how it was before?

Thx to anyone who can help!

My Setup: GPU: RTX 4070 12GB Vram, CPU: Ryzen 7 7700X, 64 GB Ram

Without a restart of the sim, you might still see effects of the previous setting, for example memory can still be in use and not released.