1997 Volkswagen Jetta Speedometer

Hi I have never seen a sender unit give trouble if you take it out of the box you can turn it with a drill with the ignition on if you keep the drill speed constant it should stay constant on the dash. Check the teeth on the worm drive when it is out, it is not in the box as the drive is steel. I know that there used to a problem with a resistor in the speedo clock circut, also Check for chaffed wiring or bad earths although I think this is a long shot in your case.