So I’m having this same issue with my foot pedal. Studying electronics at school I thought I’d take it apart and see what I’m working with. The problem is the circuit board. The sliding portion just adjusts your speed and varies resistance in current to the board. This is not the issue. It’s a pushbutton contact on the board that gets corroded and stops working. Problem is I can’t find a replacement board for it without buying a while new pedal.