What you are describing is probably not the radiator problem, or you would be running hot under load (Highway speeds). My bet would be your radiator fan (either switch or clutch), or, if you are leaking coolant, a water pump. However, water pumps on MB fails rather rarely. Above 45 mph the flow of the air over the radiator is sufficient to cool it. If the car does not run hot while driving, your radiator (and the radiator/expansion tank cap) is probably ok. make sure that your radiator fan kicks in at idle when the temperature gets high enough.
1985 190D 2.2l Sold-to Brother-in-law
1996 Mustang 3.8l -"thinks it's a sports car"
1988 Grand Wagoneer - Sold (good home)
1995 Grand Cherokee Ltd -"What was I thinking??!!"