Have your injectors been replaced with the new brass-colored ones?
I have a post in the forum somewhere from awhile ago where I reproduced my mechanic's list of a number of the possible sources of rough idle on these engines. In my 300E, the culprit turned out to be the injectors and, believe it or not, the solution, while it seemed weird, worked to perfection. I had the original design injectors in my engine and it was explained to me that the original design did not permit a good spray pattern with modern gasolines formulated according to rules adopted after my '86 was built. I was advised to run a tank of 87 octane unleaded followed by a bottle of Techron and see what happened. The 87 octane, I was told, burns hotter, which catches up all the fuel in the cylinder, regardless of how it was sprayed in, and means that there is less unburned fuel left for secondary little explosions. The Techron assists in cleaning the injectors. It worked and I was amazed. Before y'all start spamming me, my engine didn't ping or run hot, either.
I think one long-term fix is to replace the original injectors with the new-design ones. My TE has the newer injectors and idles like a brand new Singer sewing machine - one can barely hear it turn over. In fact, when I first got the car, my girlfriend drove it and a couple of times shut it off and restarted it because she didn't think it was running upon initial start-up. When I took the car to my mechanic for pre-purchase inspection one of the techs had a TE in the service bay next to my TE (2-bay area of the garage). Both cars were idling at the same time and yet it was no noisier in there than the air conditioning in my office building.
Good luck. The rough idle issue can be frustrating and annoying.
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
2003 Chevrolet Suburban