I'd start with a good cleaning and oiling of the machine. All the way from the top to the bottom and from domahickey to the thingamagjig. Then clean the motor out. My local guy told me he uses lighter fluid to clean the motors out.
(DISCLAIMER: This is a suggestion made by my local sewing machine repairman to help sluggish motors. Do this outside and be advised lighter fluid may ignite when you turn the motor on. It will burn off quickly with no damage to the motor. Do this at your own risk.)
He said, take the belt off and run the motor at full speed till it loosens up. If that doesn't help shoot some lighter fluid in it and then run it. He did not specify but I'm pretty sure he was talking about doing this on the brushes and commutator.
:-> Joe