A friend of mine once had a problem with a 1957 Oldsmobile. The fuel line had somehow gotten a tiny hole in it (aparently not a severe or noticeable leak) and was sucking air. This was discovered by hooking a compressor to the line and pumping air through it. It made a whistling sound and was discovered that way. The problem was discovered on the car's fifth trip to the shop.
Could this happen on a Mercedes? I am assuming that fuel lines are made of better stuff on a MB, and they are much beter positioned. This Olds had the line running down the frame rail, easily exposed to the elements.
Just a thought.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"