sometimes it is carbon buildup on the EGR valves that causes rough idle. my 97e420 idle fluctuated slightly when cold. i noticed that when idling the rpm would go from about 650 to 500 and up again but i don't notice that once the car warms up. my car has 152k miles.
if it is carbon deposits then the only way is to take the head top off and manually clean the valves.