If it does not overheat on the highway then it is not a plugged radiator. The engine uses a lot more power and thus produces a lot more heat when driving on the highway than idling. So if it will stay cool on the highway, the radiator flows plenty well enough to cool the car at idle, as long a water and air flows are adequate.
On my car the radiator is semi-clogged. I can drive around town on a hot day and the temp is rock solid ~80C. Go on the highway at 70mph and the temp is fine, but if I turn the AC it starts to climb. If I go faster, it climbs more. If I slow down, the temp goes down. So the radiator is at its cooling limit at highway speeds (the water pump is new and there is plenty of airflow on the highway).

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Csaba
1972 280SEL 4.5, silver