Temp gauge goes from normal to H then back to normal within seconds.

Possible air in the system. Try purging the cooling system.

Today, my temp gauge goes from normal when the engine is cold to H after the engine starts, but has not warmed up. When I disconnect the temp gauge wire from the sensor, the gauge goes to zero-no temp reading. Any thoughts? Coolant level is good and new thermostat. Fan operates, no hard start or smoke.