I would suggest that you follow Steve's advice and send it to a speedo shop.
My speedometer began doing just what you describe, and then it got to a point where the trip odometer was preventing the main odometer from turning. If you get to this point you can save the main odometer by removing the spur gear between the main and trip odometers. You will then, of course, lose the use of the trip odometer. But again, if it's not an undue financial burden, have the speedometer overhauled by a reputable shop.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in