I had the same problem yesterdayon my 85 300D turbo. when I was trying to change my defrost change over valve. I think if the vacuum line comes off the change over valves, you will not be able to shut off the engine under certain conditions based on which valve connection has come off.
Did you do any work on the car recently. Once I had a complete shut off problem( car wont shut off at all) after an oil change and I found I had accidently disconnected a vacuum line from the 4 way rubber connector. Push all the lines nicely into these connectors before you go for checking the change over valve connections.
does your door locks open and close normally? try about 10 times while the car is running. If they dont hold the vacuum then you most likely have a problem in one of the connections in engine compartment.
Good luck
MVK

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1985 300D Turbo 179,000miles