Try locking the doors when it is having trouble shutting down or lock them BEFORE you shut it down and see if it makes a difference. Your vacuum shutdown pod on the injection pump might be going bad. If you suspect the locks, only disconnect the YELLOW vac line under the hood and plug the rubber connector. That isolates the locks only. See if it shuts down better. A Mightyvac vacuum pump is real handy for these old Mercedes to find the vac leaks at the doors and other vac operated things on the car. Get one at an auto parts store.

__________________
My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.