Pete,
Is there a squeeking/chirping noise? I had this problem and had to get a replacement speedometer, because the plastic on the backside had worn out. (and I was too cheap to have it rebuilt). You can check to see if its your speedo or the gear in the tranny by disconnecting the cable at the tranny and hooking the end of the cable into a power drill. Careful! The speedo is only meant to go one way! If the speedometer increases normally, then the prolem should be the gear in the Tranny (about $5 at any tranny shop).
