I have a 95 E320 with 77K miles. It is up-to-date with maintenance. A couple months ago it started to act like it was occasionally was "misfiring" when the car was in gear but at its lowest idle (like being stopped at a light, or in reverse with the brake on). The engine normally idles at ~550 rpm at this point. During these episodes there would be a little shudder in the engine, and it would would drop to ~450 rpm for a second. It gradually became more frequent. I'm in Denver, so tempuratures were 20's - 40's during this time.

About two weeks ago I took the car to a reputable independent MB specialist (my first visit to him), to check it out and do a lube/oil/filter also. He said there were no internal computer notifications of any problem, and on further investigation (he used a couple different engine analysis machines) found what appeared to be 1 or 2 bad plugs. They had been in for less than 10k miles. One plug in particular showed evidence of carbon tracing linearly along the white porecelin of the spark plug. He replaced the plugs, and the car ran great.

A few days ago I noticed a small shudder in the engine while waiting at a stop light again. It has started to become a little more frequent; 2 or 3 times in a 12 mile drive to work. I haven't pulled the plugs to look at them, as I don't have a torque wrench for proper replacement. I'm going to contact the MB tech again, but does anyone have any ideas as to what might be the problem?