This may sound strange, however, check your a/c idler arm pulley. I had this problem in my '84 500sel. Every mechanic I brought it to could not find the problem. It went on for about six months. Sometimes the car would idle smooth, especially after being run at high rpms, then it would just be very rough at idle. If you watched the oil pressure, you would see it was not steady at all.
The bearings in the idler arm would try to seize and cause the engine to chug. I figured it out when the bearings finally started making enough noise to warrant replacement. Upon inspection of the old unit, I could see how it would occasionally seize. When the new idler arm was installed, the rough idle went away!