Have you done a thorough cleaning of the cooling system? De-oil, de-calcify, new coolant?

While you're at it it wouldn't be hard to clean the outside of the radiator and condenser.

Probably at some point you'll have to deal with the fan clutch though, if its cooling good while moving and not while sitting still the fan clutch is probably at fault. Be glad its not the 2 year only electromagnetic jobbie in my 190D...

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Sadly Benz-less