I agree it sounds like you may have a faulty pot (potentiometer). To verify, if you have a multimeter, put it on the pot wiper (middle leg) and an outer leg, and as you rotate the pot, observe for any "sudden jerkiness" in the ohm value reading. With a good pot, the ohm value reading should change smoothly / continuously. If you find the ohm value is "spiking" or jerking around, it's time to replace the pot.