was it like this previously? How long have you run it?

Do you hear the fans kicking in? Sounds likea thermo issues, maybe.

An air lock in the coolant system would give an effect on the sensor.  Its designed to read water temp, not air temp.

To ensure all air is out, from cold, put interior heater on hot, expansion cap off, top up water to level, start engine and regularly squeeze the large coolant pipe at the battery to burp air back into the header tank.  Keep running/burping until car at norm op temp then replace cap and take for a run.  Allow to cool (overnight preferred) then check/top up water level in the morning.

Should be OK then.