Car fires but won't start up after driving it for 30 minuets then left to cool for starts fine why

When you say clicking sound. Do you mean it sounds like the battery is low and the starter is attempting to turn? Or do you mean the engine cranks,(trying to start) but never fires? If its #2 then you may have a bad crank position sensor. They are a common failure point of the 113 Mercedes engine in you your Clk500. When they heat up they lose the crank signal. When they cool back down they work fine.