Along the same lines, my 80 300SD has started exhibiting this problem.
I haven't had yet started diagnosing it. But here's a clue. In
winter, it will only shut off if I've been driving the car for a few
minutes or run the engine for 20 secs or so at higher RPM. If it's
been idling, then it won't stop. Now that the weather is warmer, it
works fine again. Also, no other obvious vacuum problems, the door
locks will work days after the car is shut off.
Any guesses as to whether it's the bellows or a vacuum leak somewhere?