puff of smoke and fluctuating idle

[chiefsittingduck]

Hi all, ive had so much grief with this C200 of mine!! the EML is on again! i started her up this morning and noticed on tick over she fluctuated a bit - not much but enough for the rev counter to move up and double a couple hundred revs and enough for me to hear, as i pulled away i noticed a small puff of black smoke from the exhaust - something i haven.t noticed before!!
this is all on top of the EML light coming and going following lots of EGR and sensor issues... the last time it went to mercedes and they replaced a sensor they said it could be a build up of soot if the light comes on again... which it has...

any thoughts!!!!

thanks in advance