Ah… that is probably the famous CPU LOD bottleneck. In short, your Terrain and Object Level of Detail settings as well as traffic settings (especially airplanes) might be just a bit too high.
LOD seems to be sent to the last CPU core, and if set too high, it tends to max out the CPU, especially at big airports. And if one core gets maxed out, the whole computer is throttled to keep all cores in sync with the slowest core.
When the GPU is the bottleneck, your FPS slows down. When the CPU becomes the bottleneck, you get immersion breaking stutters (and those are terrible).
You may want to check Dev Mode in sim, and follow the steps listed in this linked post and subsequent posts in the same thread.
This thread talks about maintaining a locked 30 FPS but if you have the system for it, you can maintain a locked 60 if you go for 15 ms instead of around 30 ms.