Make sure the belt isn't too tight. Just tight enough so it doesn't slip is tight enough. Any tighter and it will drag down the motor.
The sparking could be due to dirt, lint, and old lube on the commutator. Were it mine, I'd double check the belt, loosen the stop motion knob ( oil the hand wheel shaft first ) then run the motor wide open for a few minutes to see it it would clean itself out. Not telling you to do that, but that's what I'd do.
You might also take the controller apart and clean out all the stuff in it. Make sure the wires are tight and the contacts are clean.
Joe