Mine did the same.
Some people fix their by taking the cover off, 10 small screws and 3 big ones, and then cutting the black earth wire that connects to the motor (the one next to the blue and red wires).
That didn't work for me.
I fixed mine by cleaning the brushes on the motor. You have to use electronic component cleaner spray, not wd-40 or lubricant. The brushes get sticky or loose contact, and it just needs a cleaning.
Just stick the nozzle in the side slits of the motor, aim upwards, and spray both sides. Wait 3 -5 mintues for it to dry, and then try it. It might take a few turns and maybe a respray. It all depends on how dirty those bushes were...
Now mine works even better than when I got it 2nd hand...
