I have electric fans on the Galaxie and was having problems with the car running to hot.  It would get to a good but slightly hot temp then the mechanical thermostat would open but there was a slight delay (about a minute) before the fans would come on making it get even hotter especially at idle.  once the fans would come on the temp would drop but then the mechanical thermostat would close causing the electric thermostat to shut off the fans and the entire process would repeat it self.  In other words the gauge would swing from the middle to the high end then work its way down and repeat when not at highway speeds.  

The solution was to eliminate one thermostat.  By ditching the mechanical thermostat the radiator gets hot with the engine and the electronic thermostat is more accurately coming on in retaliation to water temp.  The gauge sits at about half most of the time with only slight swings up and down at idle.  It runs just below half at highway speeds which I am still very happy with.