My 99 SLK seems to be misfiring under load in second gear at about 20-40 mph. It lights the check engine light when this happens. I have had the coils and spark plug wiring replaced as well as the air flow meter. Each fix seems to make it run better but doesn't seem to fix the misfire/loss of power problem. Any suggestions?

 

Take it to someone (mechanic) who can read the trouble code for you.