A thermostat sets the minimum operating temperature of an engine. The efficiency of the radiator is supposed to maintain that temp once the thermostat opens. If an engine has a tendency to overheat once the stat opens, it's the fault of the radiator, or deposits/debris in the water jacket that interferes with good heat transfer from the cylinder walls and/or cylinder head to the coolant. Having enough heat transfer from the coolant to the outside air to maintain the proper engine temperature is the purpose of a cooling system- - - -the thermostat is only one link in the chain.

Trivia- - - -if all the heat energy released by the burning fuel could be captured and applied to powering the car, gas mileage would triple. Approximately one third of the heat energy goes out the exhaust, and the cooling system transfers another third to the atmosphere. The one third that's left over is actually what's used to move the pistons.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!