I ran a Diesel Rabbit for 13 years and learned a few things that may be applicable here. I would check the glow plug on each cylinder to see that they are operating. As diesels need a bright heat source initially to start a non-functional one would cause these symptoms and would go away shortly after start up as the compression quickly builds up enough heat to sustain the ignition of the diesel fuel. It would not explain the fact of only occurring after a few days of non-use. I also assume that the "over engineering" aspect of Mercedes probably would detect and alert the operator of a non-functional glow plug. Rabbit's diesels were the most simple of basic diesel engines - 4 high compression cylinders and injectors an injection pump fuel filter and glow plugs. Those are the basics and besides oil and good 590 CC Amp battery that is all that is required for a good running rabbit engine.