What type of intake do you have? If it's the original, I would change the air box seal that connects to the MAF and tighten the brackets that connect to the bearings (rubber bumpers on the valve cover). I found mine not clipping on to the rubber bumps tight. That caused the airbox to raise slightly enough for to much air to enter. I had suffered with the High RPMs for a while. Went through 4 or 5 MAFs. Each time they lasted for awhile and then high RPMs. I never thought about the seal or the airbox lifting up enough to cause to much air in the intake manifold and cause misfires. I installed a new seal on the airbox 2 days ago and it drove a lot better. Today I did an inspection and found the airbox slightly lifted off the MAF. I had suspected the rubber bumps and swapped the ones closest to the MAF to front of the engine when I installed the new seal. Today I crimped the metal brackets on the airbox that clip on the rubber bumps on the valve cover. I will test it in the morning. Follow the link below to see the clips on the air. They are item 287 in the picture. Lots of part numbers for the SLK 320 on this sight. I ordered the seal from Needswings.
https://www.ilcats.ru/mercedes/?func...=4&language=en
Part number for the seal on the airbox and rubber bearing
A1120900089 Intake airbox bearing
A1120940180 SEAL MAKE MANN & HUMMEL

I also changed the 2 seals on the MAF elbow. One at the end where the MAF connects and the one on the end where the elbow connects to the throttle body.