i have a b & h 8mm projector model 256 that won't stop "clacking"?

it threads great, projects great, and will get part of the way through the reel and will always start clacking (or whatever you call it). the frames are off, etc, and you cannot see the image. pushing on the threader will not reset it. it will do this every reel, and at random places in the reel, but not always at the same place so i don't think it's the film. i can remember my dad running this thing for hours with no problems. what gives?