I have the same model and I never noticed this behavior. Doing some quick tests, if I turn the Vertical Position encoders (I don't think they are potentiometers) really fast, the cursor sometimes does not move linearly, in a constant speed, but it never moves in the reverse direction.
So it could be that the encoders are missing positions, but I also think the scope is not that well designed to allow for fast movements of the knobs. I really could not say.
I already replaced the power switch on the top and the cpu fan, but I don't have a inside picture to see how the encoders are mounted and how difficult they are to replace, if they turned to be deffective.