It sounds to me like you may have a bad plug connector. These can go bad on any Mercedes gas engine from 50K miles on. The easy way to find these is on a scope. Other than that you can carefully read plug color and often find the culprit, I find this particularly useful on multi-coil ignitions that are difficult to connect to a scope. Other than that a vacuum leak could be a problem but I can't see it going away. If it returns pour water over your running engine an look for a reaction.
Good Luck with the hunt