Started experiencing an issue with my 2002 SLK230 with manual transmission. With clutch dissengaged and without pressing on the gas pedal, the RPMS rev to about 2K when foot is not on the brake pedal. Upon stepping on the brake pedal, the RPMs settle to the appropriate level. Let the foot off, the engine revs again. Revs don't decrease when shifting between gears either. Anyone have an idea what is causing this?