The symptom you describe is usually caused by a thermostat that is not closing completely. It is the job of the thermostat to shut off the flow of coolant through the radiator to keep engine temperature up to normal (195 degrees F). If the engine temp is dropping below normal when the vehicle is moving and air is flowing through the radiator core, that indicates the thermostat is not doing it's job and is allowing coolant to flow through the radiator when it should be closed.
Replacing the thermostat will cure this problem.
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