The thermostat can fail in many ways, one of which will cause a failure at low speeds only. That is because it is only partially open. At high speeds the water pump is forcing the coolant by and therefore cooling OK. Replace the thermostat first. Then the radiator if not fixed.
Don't overlook the cap.
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM