This maybe due to new "square" brushes settling into the older "worn to a shape" type armature, if sparking still occurs after a bedding in period (which maybe seen to go round the armature in rings) then suspect a defective winding in the rotor.
If you can run the motor (minus the blade) and get access to the armature you can use a comm. stone to clean the armature and bed brushes simultaniously as the motor runs.
If a winding(s) in the rotor is defective it's generally a bin job
Jac Flash
July 2008