How did you check the pot? The best way is to measure the wiper voltage (middle pin of the pot) with respect to 0V (ground) and wiggle the pot. The wiper voltage should go from -15V to +15V as you rotate the pot from one end to the other. If you can't get to the wiper pin directly you can measure pin 3 of U16 instead.
From what you are saying I would suspect the pot first. Sometimes the internal connection from one of the outer pins to the resistive track can break or become intermittent. This damage often occurs in transit when suppliers put the pots in a big bag all together.
Another part that could be faulty is U16.