How rough is it? We have the same model year. Mine stumbles through idle, but it's very subtle and not much of a problem. I second what JCE suggested that you check. In addition, you may also have the original injectors (like mine) which apparently don't like premium gasolines as they are currently formulated. There is a thread in which I discussed what my tech told me about rough idle in my 300E and what I tried (regular instead of premium) as an experiment. Do a search under my username and you'll find that thread. The guy who started that thread said that his injectors were replaced and ultimately the fix on his was as simple as JCE's (as I recall). If your idle is REALLY bad then I may be running off without my mouth connected anywhere sanitary and I apologize.

__________________
David

His:
2003 Dodge Durango SLT
2002 BMW E39 530i Sport
1988 Mercedes 300TE
Hers:
2003 Chevrolet Suburban
1999 E430