Early machines used a dedicated controller board for the deck motor, later ones use a programmed inverter. Sometimes the inverter back up battery gives up and the programming of the inverter is lost and returns to default values for acceleration, motor current etc. Deck rollers are a possibility if it's an old machine, the bearings are replaceable, however they are usually a press fit to the shaft which will make it awkward to do at home. It's usually dead obvious if they are shot, it will rumble like mad and the shaft will bind when turned by hand. The other possibility might be that the deck is dry. Most machines use a phenolic type deck and require a little lube now and then to stop the track binding. This isn't just normal oil, get the right stuff. If you disturb the deck rollers, be sure to mark the distance from the frame that each tensioner is set at, otherwise the track will run off to the side and foul the frame, the rollers must be parallel to each other for the track to run true in the centre of the deck.
Hope this gives you a couple of ideas...
rgds