I read a post once on this forum that described your exact same problem. Something about the paint degrading and becoming sticky. Search for it and find it. The fix involved removing the speedometer, cleaning the post off, and painting it with super glue I think (keeping the needle off of it until dry of course) You'll find it if you search...
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick