The V-Pots on both of my d8b's were all pretty much unusable when I got them. Not just the one's below the display, but also all the ones on each channel. Calibration didn't solve it.
I cleaned all of them with "Deoxit Fader". Sure - they do feel a bit looser afterwards, but at least they're usable.
I would think that the V-Pots would slowly develop issues where they start hanging and jumping, etc., and not just start doing that after a different sounding issue (audio issues with jumping faders in the GUI, like this thread started out).
But if that's the only thing, I'd definitely try cleaning them (or first try calibrating... forgot if that's possible for those pots, also).