Speedometer Problem
 

I have a 1983 Mercedes 300SD with a speedometer problem.
Sometimes it works fine but most of the time when the car starts it goes to about 40mph and pulsates there -- maybe a little higher when going faster. The odd part is it pulsates a lot more "in time" with the directionals when they're on. It acts like the speedometer is electronic (I know, everyone says it's mechanical) with a bad ground. I saw something like this posted once before and everyone said it couldn't happen but it does.
Cruise used to work when speedometer was working now doesn't work at all.
Help!:confused: