This is a "toe dance" . . You have to flow enough coolant through the radiator to cool it to 140° at the bottom or inlet hose on just about any gasoline engine being made today. The temperature going in at the top is a moot point, but the bottom is very important.
Dunno how big your fan is and how tight it fits in the shroud, but normally, unless you have an engine or a flow problem, a 15"-16" flex fan with about a half inch clearance at the fan tips to the shroud should cool it efficiently on both ends of the rpm spectrum.
It sounds to me like your raditor isn't getting enough air to it at low speeds. Or, your radiator hasn't enough capacity. I say that because if it was too much flow at low rpm, you should have heating problems at much higher rpm, or all the time.
Rule of thumb is; "The more coolant in the radiator and the longer it's in there, the cooler it is on re-entry to the engine" . . .