A partly plugged up radiator is about the easiest of our cooling problems to diagnose. With a tempo gun, a simple scan of the radiator surface when warmed up will show any significant temp differences. And compromised water flow could impact the engine's temp at any speed. But if your over temp issue was only at low speeds, it is hard to see how it would cool okay under load and at hiway speed. Both of which puts more load on the engine and cooling system than idling does and would require more water flow.