Dan,
It would have helped if you posted a photo. The obvious cause of the chatter is the placement of the fiber or insulating pins is incorrect.
next, I found in an excerpt of a repair manual for the older style, mention of the sliding pieces internally grounding and causing chatter.
frustrated as you may be, there is a repair. I don't run these, so I can't guess which one that you have. They are simple and can be made to work.
