Have been working to resolve rough idle for 1.5 years now.

From time to time, I seemed to have gotten it under some control (w/use of different gas/techron/lubro moly products), but lately it had gotten worse.

Although I do much of my own work, I do have a very good local independent mechanic (who bails me out when I get in over my head) and who offers free inspections once a year. I had been taking my wife's car in every year for this check up (wife cannot tolerate the thoroughness her in-house mechanic applies when performing repairs - considers him too slow), but this year decided to take my '90 300E in. Mechanic noted 3-4 problem areas - including worn motor mounts. I had, of course read about this as potential cause of rough idle on this forum, but had been seeking electrical/fuel related solutions, so had not yet considered this.

Since my idle problem had worsened over time, I asked him to go ahead and replace the mounts, and the transmission mount.

Amazing change! Amazing improvement!

I can barely tell the car is running now!

In case anyone else is at wit's end on this idle issue, you may wish to give another thought to those motor mounts - they really do make an incredible difference!