My old (1971) Clark EC500 has developed a new issue. After changing out the hydraulic motor and pump for the mast, the forklift no longer moves very fast. Battery voltage is up and everything works normal until you touch the accelerator. You can only touch it very lightly and as you do you can watch the voltage drop to the red zone in a few seconds. You can hear the motor spin up and it moves forward very slowly. The same happens in reverse. Stepping on it even more and it just stops. I'm thinking the potentiometer is shot but I have no idea why it would suddenly go.