i would say the maybe your speedometer gear has no retaining clip, so its kinda sliding around in there. or your speedometer cable is somehow loose in the back, which could rarely be the case. there isnt too much that can be wrong with a speedometer. it goes from the tailshaft drivegear, that spins the speedometer gear, which transfers throught the speedocable into the back of the speedometer. something is loose in that little system.
to tell you how to look at your gear, i will need to know if you have a AOD or a T-5. you should fill out your sig so everyone knows whatcha got.