I had a similar problem with my 300 SEL. I replaced the injectors, went throught the fuel system extensively to no avail. The soultion for me was to adjust the mixture screw at the top of the fuel distributer. This should be done with an exhaust gas analyzer. However, if you have a mechanic set it up based on factory specs it will probably still stumble. Used the trial and error method and solved the problem. ( you can't do this if you are in a state which tests your emmissions system during state inpections)