Thanks MB.DOC. My SLK has a non-compressor engine.
I asked my mechanic after Your replay and he confirmed that the Bosch spark plug type F8DC4 has been used.

Regarding spark plugs, are there any other alternatives for the SLK200 non-compressor engine?
I am asking because lately I have experienced engine misfire.
My SLK works great when cold but at operational temperature the engine hesitates a little bit and misfires under load. The misfire region is from idle (500-700 rpm) to ~2000 rpm. Above 2000 rpm the engine is working well. If I accelerate “carefully” the engine do not misfire but a short hesitation could be felt initially. After a deceleration to below 2000 rpm followed by a fast throttle action the misfire is accentuated.
The air flow meter, O2-sensor (Lambda), both coils and the spark plugs have been replaced. No error codes has been seen.
The strange thing about this misfire problem is that with new spark plugs (F8DC4) the engine was working very well for a short period of time (five days).
Then coil no. two was replaced, still using the “old” spark plugs. The coil change did not cure the misfire but some improvement was observed.
The throttlehouse has been cleaned. Redex (injector cleaner) and STP (Complete Fuel System Cleaner) has been added to the gas with no effect.
Could a defect or poor coil damage the spark plugs? If so I will replace the “old” ones with the same type or do I have some other options….?

In advance, thanks to all that cares to answer. I have read a lot about misfire in this great forum but it is difficult to find cases that “exactly” match my problem and as a consequence (with my knowledge) to find the correct solution.