Hmm, that sounds really awkward. Have you fixed it already? What caused this kind of an issue? My dad also has a JLG 600S, and we have been using it for more than 12 years, and we have never faced a similar problem. I cannot even imagine what could actually cause such an issue. I do not think there is anything wrong with the pedal, and there must be something else. Maybe the engine or something else, but surely not the pedal. If you haven't fixed it yet, you could call for the services of https://anguslifttrucks.co.uk/ as they are probably the best in forklift repairs. We do our repairs only there.