Just to update this, it was solved. The original poster asked this question in the Facebook group where he solved it by cleaning the sliders and knobs. Judging from his statement that he is simply using this as a piano 90% of the time, I suspected the sliders and knobs are rarely used, if ever, which could lead to erratic operation. This is simply how any potentiometer behaves with age, certain environmental conditions and lack of use. I always suggest simply turning the knobs, sliding the sliders, and turning the wheels occasionally. They can clean themselves and remain in good condition.