750 RPM, I hope! Rough idle seems to be one of those wild-goose-chase things since the cylinder is where it all happens. I would check electrical next--often a few of the resistor-tips of the spark plug wires need replacement. Let us know your results, as many of us (including myself) are tracking the elusive rough idle. Also invaluable is going to an MB tech and having an oscilloscope test. The trace seems invaluable, as it is described on the CD-ROM anyway. Best of luck, Andrew Seidel.
89 190E 2.6
83 300TD Turbo