[Ted in Toronto]

It sounds like classic carburetor icing, but you don't have a carburetor. When cool humid air passes through a venturi it drops in temperature, and the humidity forms ice, blocking airflow. Before the days of fuel injection most engines had a thermostatically controlled flapper that mixed warm air drawn from over the exhaust manifold with the incoming cold air to prevent this. I don't know if the SLK has some form of intake air pre-heat system but if it does I'd look for a problem there.