I have now had the car to my regular mechanic, the Mercedes dealership and a mechanic that came highly recommended on this forum. Nobody can find the problem. The mechanic I found through this forum says the only thing to do is take the injector pump off and send it to an overhaul facility. He will ask them to check the pump before performing the overhaul, if the overhaul is done this is going to cost over $2,000. We did replace the fuel pump, I guess that is what people call a "lift pump" on the forum although the mechanic did not know that terminology. Had no impact. He also replaced all the fuel lines, again did not help.

Does anybody have any other thoughts?

Thanks,
John