You mentioned in reply to @Ar9Jim that you would get the distortion at idle while sitting still and when adjusting the volume. That may indicate that the issue may be more/other than the pot. As cleaning a dirty pot as you've now done is pretty often successful, perhaps it's time to get the whole unit to a skilled shop for repair? An added benefit is that you could also have the whole unit checked against spec, service bulletins applied, and any questionable components replaced so that you could get many years of enjoyment from it.