It's most likely your "fuel accumulator". I have the exact same problem. It's easy to check. Just remove the clamp securing the fuel accumulator "drain line" to the fuel damper, but first make sure you've got something to press against the opening to keep the fuel from the tank from shooting out of the damper. I used a piece of hard rubber and just pressed it against the damper. With that line disconnected, start the car. If fuel comes out of that drain line, the accumulator is bad. The Mercedes manual calls it the "fuel reservoir". It's a simple mechanism comprised of a diaphragm and a spring. It's hooked up to the pressure side of the pump. The fuel pressure pushes on the diaphragm and compresses the spring. When you turn the car off, the spring gradually releases the fuel pressure "to prevent vapor lock and to help in hot-starting the car". What happens is that the diaphragm starts to leak. The drain line is there just for that purpose. It routes the fuel back to the fuel line so that it doesn't leak out onto the ground. Believe me, I had the "exact" same problem you've described. The thing is that when the diaphragm starts to leak more, the car will die on you while you're driving it, since the leaking diaphragm is lowering the overall fuel system pressure. The accumulator is located above the fuel pump and the fuel filter at the rear of the car. There's a hard fitting on one end, and a standard rubber fuel hose at the other end. That rubber hose is the drain line, and the bottom end of that line is connected to the fuel damper with a small clamp you can remove with a screwdriver.