How can I fix my Bell&Howell 8mm multi motion projector?

Could be grease, could also be any rubber parts, rubber hardens over time, might get away with a clean with Isopropylalcohol (IPA).
I would suggest, give the m/c a good clean, take bits apart and clean old grease etc off, IPA can do that as well If it uses rubber belts to drive the film spool, as a lot did, not just drive but also to keep pressure on the feed spool, so it did not over run and spool film unto the floor, check them, replace if needed, any rubber band about same size would do the job.

SD
December 2011