My rpm's drop during acceleration. I have a 02 c230k sport coupe 2.3litre. When my gauge reaches 4000rpm it drops to idle so I have to drive below that.

 

Unfortunately, that is just a brief, meaningless statement. If you have a problem, and are trying to ask for assistance, you will need to do a much better job of doing so. A clear statement of what happens, when, how often, how bad, etc., etc., along with the model and year of your car (and this Forum is pretty much limited to W220 problems), and your location (W220 specs differ from market to market) would be a good start.