First off, welcome to the forums and to the SRT family.
The coolant temperature sensor is near the top of the cooling system. Typically if you have any air in the system that has not been bled properly it will collect up near the sensor. Hot air will read hotter on the gauge then hot coolant. While changing out the thermostat is probably a good idea (if you don't know the age on it.), flushing the whole system and ensuring it is properly bled likely will do more for your situation.
let it idle for 20 minutes (keep eye on temp) and look under the car for a leak.. most overlooked are the 2 coolant lines to the oil cooler and the 2 coolant lines under the intake (connected to the back of the water pump)