Mine did the same thing when new. My dealer thought it was the EGR valve sticking open, but when they disassembled and reassemble the air intake the problem was gone,over three years now. I've read on a couple MB forums it could be the EGR, mass air sensor or any leak on the air intake. Look for any loose hoses or clamps on the air intake. You can test the EGR valve by disconnecting and plugging the vacuum line to it, thats what my dealer did.