missfiring

I have a big problem that my main M/B agents do not know what the problem is! , C Class 200, Komp, Petrol 36000 miles from new, car runs like a dream, perfect UNTIL I sit at the lights or a traffic jam, the engine starts to missfire as if running out of fuel, or as if someone is taking the plug...

Just been out and about most the day in the SL on the way home from Birmingham, i noticed it was a little lumpy on acceleration. after a while it did seem OK then it came back??? I had to replace a distributer cap a month or so ago, should i of replaced the together? whats the best way of...