Whenever I get desperate, I take a copy of my project and slowly tear it apart, like delete sections of it (maybe whatever event sheet groups that use the most CPU in the debugger preview), and test each time, to see when the stuttering stops. You might find some specific feature is causing the jank.
Recently, I tried my project on a 4k monitor, and I noticed that even tho I had solid 60fps or 1000fps when vsync off, it was still jittering badly. Turns out it was me "pasting" on drawing canvas every tick, it didn't like that I was doing it every tick on a 4k sized drawing canvas. Obvious now that I see it, but I wonder why was my fps was seemingly fine.