Following Steve's advice on a possible vacuum shift problem, find a knowledgable MB Tech who knows this tranny and have him do a pre-purchase inspection. May cost $100.00 or so, but he will be able to give you a good idea of the cars condition and possible problem areas.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.