The most common cause for the ribbon to stop spinning intermittently is a worn spring: the spring that maintains constant tension on the nylon wire that drives the ribbon-drive gear assembly. If the spring is worn, the constantly-varying tension within the ribbon cartridge (transmitted to the gear assembly), causes the nylon wire to slip.
Try replacing either the Ribbon Wire/Spring assembly (service part number 935-0001) or the spring only (available as a part of the Miscellaneous Hardware Kit, part number 076-0317).