pyukumuku Dude, did you try doing what I said. The jitter is caused by your update rate not being in sync with your frame draw rate. If you update in _process() you'll update for every drawn frame and there should be no jitter regardless of monitor refresh rate and vsync setting.
Since you update manually you don't depend on physics engine and you don't need to do it in _physics_process(), which is called on per physics engine tick. If physics engine tick is out of sync with frame rate - you'll get jitter.
That said you should worry about this once you have a bit more on your screen than a single sprite.
