I think it's normal for brushes to occasionally spark, but this is excessive. If you want to fix yours, take off the brushes and clean the area where the brushes meet the motor, both on the brushes side and the motor side. Also, take the brushes out of their holders and clean the holders. Make sure the brushes can easily slide inside their holders. Add a slight chamfor around the edges of the brushes that touch the motor shaft, and reinstall.