I spent $8600 trying to fix a rouch idle in my 300E 1987....and I'll admit many other things (sunroof, AC, Alt, Brakes, tires, rims, basically entire d&*m car)
When the tach jumps up and down, then its probably related to the fuel system (I've had the problem too) I couldn't tell what it was for the last 2 years and neither could any of the MB mechanics. In my case, the rough idle was most pronounced in D and R, but 50% reduced in P or N.
I noticed the motor mounts were basically flat last week and had ASC in Arlington VA replace them. Car has never run better. All the engine vibration I felt at idle (when stopped) went away. Car purrs like a fat cat!!! Its awesome....
PS: excuse my html code below, i'm trying to convert to VB