John you may find that the worn brush and loose connection has sent spikes back to the controller over a period of time and stressed some components on driver board. Knocking may have been the instantaneous changes in speed. It also may have damaged some segments of the commutator or has carbon shorting out in slots. You can clean segments out with a groundown back of hacksaw blade and run along slots to remove carbon. If commutator worn you can lathe down commutator and scrape out the insulation to below segments using sharp end of hacksaw blade as mentioned above---old trick used in automotive industry before throw away society became the norm. Can you check if commutator still arcing with new brushes by leaving the cover off to test. You may be able to reach in and clean slots without disassembling motor.