Dave,

need to know what engine this has in it. 111 engine silver cam box or the 271 engine black cam box.

I suspect it is the 271 engine and these are notorious for burning out valve seats. The fact that you can switch off the engine and restart it and its ok for a short while points me to that conclusion.

Mercedes had a software flash for the ECU that knocked out the pick up of the sensitivity of misfires. This can only be done with a Mercedes STAR machine (genuine one) but obviously the head needs to be repaired first of all.

hope this helps in some way