Failed spark plug wires and/or ignition coils can cause intermittent misfires (rough running); the Check Engine Light may also illuminate. Diagnoses of the ignition system will be necessary to determine which components are at fault.Google+
Car Problem Reports
Mercedes-Benz SLK230 Bad Spark Plug Wires/Ignition Coils May Cause Misfires and Possible CEL
Mercedes-Benz SLK230 Problem
Average Mileage: 99,500 mi (84,000 mi - 115,000 mi)
Visitor, 2004 Mercedes-Benz SLK230, 115,000 mi
The engine started to misfire at times. After a cool-down it would again operate OK. The check engine light would illuminate. Checked with an OBII tester and it indicated a misfie on #3 cylinder. Pulled the coils from #2 & #3 cylinders. Checked them with an ohmmeter. #2 had radically different reading than #2. Got a new coil for #3 cylinder. Everything now seems OK.
Visitor, 2003 Mercedes-Benz SLK230, 84,000 mi
As described, MB dealer repairs - parts $740, labor $385 total $1,125.