In 87, the VDO speedo was cable driven and the odometer is a clockwork gear driven mech. The VDO odometers of this period, in MB, Volvo, Porsche, etc., are well know for their tendency to strip or fall out of alignment, especially when owners would reset to zero when the car was in motion. You may have a busted cable than runs from the back of the cluster speedo drive to the transmission.