Kbase Article of the Week: ImageWriter II: Worn Spring Affects Print Quality »

Apple Support:

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.

Remember when Apple made printers?