I'd do what Brapple suggested.
Other than that my coolant temperature sensor went wrong last yr showing all kinds of weird readings (it is the sensor in the thermostat house looking DOWNWARDS - autozone calls it as 'coolant temp. SWITCH' - what they call sensor is for the ECU and it is above the thermostat). So it can be simply the sensor.
I also had a (incorrect temp. or simply broken) thermostat once causing similar symptoms.
But the first thing you should do is a good flush.
Tiv
