My 830qt does the exact same thing. I hooked up a DC variable speed controller to by pass the electronics in the onboard display and it does the same thing. I also took the motor apart and cleaned it and also check the brushes. I think there is a short in the motor. I am currently looking for a new motor, but not sure if I want to invest the $180. Sorry I don't have an answer for you, other than a new motor.
Ron, March 2010
