Try contacting these guys:
Nationwide Car Radio Repair and Speedometer Repair
They're good guys, and know what they're doing. Adam is one of the techs there, and will help you out over the phone and advise you what it might be. The owner is an interesting guy, too....he makes a lot of the parts he needs to repair old instrument clusters, when the parts aren't available. I needed a small gear to fix my 92 odometer, and got the part from them. When I reassembled it, there was another issue with it which they fixed for me at a very reasonable price.
