your cooling fans,or maybe something else,like a fan control sensor.chances are if the guage is reading it isnt that, but if you have a book, it should describe how to check resistance on the sensor.
Bad head gasket,cracked head.A bad temp sensor will not make it run hot.Also change the radiator cap and the new thermostat can be bad out of the box and make sure all the air is out of the system,so take the radiator cap off and let it for a while.