Unfortunately those symptoms are so common, it is hard to point out what the problem is, mine turned out to be an air leak under the intake manifold, the gaskets were all brittle so I replaced them, and it was fine. Regular suspects are :
Vacuum Leaks, injectors etc. Contrary to what many mechanics may say I have learned from about 3 merc techies that ignition wires are not really a replacement item, if anything it is only the ends that are replaced if bad. After trying to figure it out myself I decided to just take my car to a good Mercedes techie, and he finally diagnosed the air leak. key word is a good merc techie, I had taken the car to 2 guys who were "merc techies" but they couldn't do anything, all they said was the regular stuff, this part, that part, that part etc etc etc, I was smart enough to know though. Hopefully there is a good one in your area. I don't know about dealers either.

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**A few Hondas etc... etc... Not Worth Mentioning**
83 Euro W126 380 sel Money Pit (Sold)
91 W124 300e 2.6. (Sold) 3.0 Motor is Much better!!
93 BMW E34 Touring Wagon (Sold) Sweet Car!!
91 300e (Just 200k with Ease.)Best car ever ???(Sold)
90 420sel (Black with Tan Interior) Best car ever!!!!!!!