A search of Speedometer should lead you to a discussion of a loose metal gear that is attached to the mechanical speedo cable. Cure seems to be knurling the shaft of the speedo drive cable with plier teeth and press fitting the gear back into positioncausing the gear to turn with the cable rather than slipping as the cable rotates. If you can't find the info on this site it may be on the mbz.org site of Stu Ritter. If I remember it is fairly easy to do if you are careful and patient. It is tricky because you have to open up the back of the speedo and you have to secure the odometer and trip odometer number wheels with tape when you remove and return the gear/cable. Check it out and good luck.