It might be time to shoot a bit of contact cleaner into the motor brushes. You might also consider doing a full tear down, and check all the bushings and universals for dust bunnies. And while the wheel treads might be clean, you need to also clear out the axle cups in the side frames, which can build up carbon, oil, and other detritus.
And don't forget a dot of oil on the motor bearing where the shaft comes out of the motor housing.
These steps have revived many a shelf queen in my retro fleet.
And helpful hint, get a flat cardboard box, such as a soda flat or a carton that might hold cat food or some other product, and use that to contain the little parts while you work on disassembling the drive.
Good luck!
Lee
