Thanks for all the responses. The Mercedes guy called earlier today and said it is fixed !! The German owner of ******** Of Union, where I took the car, had been driving it whenever he could, to see it would quit on him so he could find the problem, and it finally quit running while he was driving it. Problem solved or so he says.
It was what he called the ignition module. It was actually what looks to me like the back half of the ignition switch. The part only cost me $20. and he gave me the old part. Labor was considerably more, but if the problem is indeed fixed, I'm happy.
Thanks again for all the responses.