I had this happen in my ML320 (same engine). The problem was first noticed when I came to a stop at a light and it would almost die. within a short amount of time it started to occationally die. A week or so latter if bacame very hard to re-start and the check enine light came on. It urned out to be the crankshaft psoition sensor which is located at the rear of the eingine just behind the throttle body. this is easily fixed yourself if you are out of warranty.