Hello !
I actually have the same problem: fans wont go off... or are very long to stop. And my rad is cold all over; but seems hot just around the fan switch (and the hoses are hot too). It sound like thermostat closes when engine's temperature become cold enough but water flow just enough to keep the temperature around the fan switch too hot to stop the fans...
You have to know that all is new in my engine bay : rad, fans, wiring, engine, thermostat, fan switch, etc... and my engine is never overheating.
Thanks for your ideas.
Best regards