Have a motor shop put armature on growler and check for bad windings.

Turn a badly arced or out of round commutator on a lathe. Then undercut mica and slitely bevel edges of commutator bars. Standard DC motor maintenance from any book. I would not use solvent on any electrical soap and water with alcohol chaser and oven dry has always served me well on 250 VDC and 450 3 phase motors.
Dan Bentler