My 310R with factory radiator is rock solid at just over 80 degrees on the gauge. Winter, summer, road, track - always the same unless in standing traffic in which case it goes higher until the fan comes on. My manual says "the needle will rise from the lower (cold) mark to the middle part of the gauge, where it will remain while the engine is operating at its normal temperature." I think mine has only one temperature sender and the ECU controls the fan. I think the Duratecs run hotter. I wonder if yours has a different radiator which over-cools when the thermostat opens?