If it’s sensitive to capacitance, it could be that there’s an issue with the capacitors on the board– in which case a small change in external capacitance could cause a large shift.  Check to make sure that all of the caps are in the right places, with the right orientation (for the big electrolytic), not cracked, and soldered cleanly. And again, see if you can stimulate a failure by tapping or prodding any particular component. Try to use something like a plastic pen to tap them, to minimize the capacitive issues.