Same problem , same solution , Thanks a lot!!

Thanks, it was the encoder gear on the motor.

I assume you checked the set screw holding the throttle shaft of the control handle?. next is the pot itself. A tab holds it in place. If loose, it will do what you describe, however, you might not pass self test on start up as the pot is out of range.
Next is the encoder belt (on top of drive motor). Check the gear wheels that drive the belt. If loose, they give false readings