I have had this issue for about 6 or 7 months and thought I had it figured out, but it came back. I have replaced everything you guys mentioned it could possibly be and it has not fixed the problem. Fuel pressure is fine- new fuel pump, fuel injectors, plugs, wires, coil packs, Cam positions sensor, crank position sensor, crank pulley, temp coolant sensor. Fuel pressure test, vaccum test, compression test all to no avail. Which leads me to believe it's in the exhaust system. Either the cats or the o2s. Could be back pressure from the cats, I did run across a thread on the MBworld forum that said it was his intake manifold. I have not had time to change the cats or the o2s, but I plan on it in the near future as well as change the intake manifold. I can say this has turned into a hobby, but I don't mind. Anyway, happy hunting. If I figure it out, I will post an update. We might need to compare cars to see if we can find a common factor that might be causing the issue. Oh, BTW mines now has 406,000 miles.