This sounds like a really bad dealer, to bad they can't man-up and stand behind their product, and by product I don't mean just the Jungheinrich but their service. On the point of your problem, I was checking out the Curtis web site on their model 1234E controller and noticed that it has inputs for two accel. pots. If this is a 1234E controller and the truck has two pots, the second one might be bad thus explaining the abrupt curve in acceleration (first one working ok and second one WFO) that said I do not have any experience with this controller or truck, just a thought.