Oh, the hours I've spent trying to figure that out...
I've bought faster, more powerful computers over the years. I've overclocked, and underclocked my gpu. I've tried cpu priority and clock speed management tools. I've added more and faster memory. I've tried Windows standby memory list managers. I tried Gsync. I've tried FPS limiters. I've tuned the Composite Budget files to match the capabilities of my computer and display. Everything helped, but nothing made it completely go away.
It's when I tried adding objects to the preload.xml in the hope that the hitches were from loading their data for the first time, and it didn't help, that I realized that something was fundamentally limited in the code.
The best that I've come up with is that the memory management for screen blitting is hard coded to use a certain amount of memory, and then does a reset when it reaches the limit. The inevitable hitch in the display is less noticeable when refilling the memory to calculate the next screen involves very few objects. It's completed so quickly you don't notice the delay. Alone at high altitude looking forward from a static cockpit view (almost no blitting needed), and it looks smooth as silk. Going fast, down low over a city full of buildings or a forest of trees, while looking at yourself from a side view is the absolute worst.