Fiat Punto temperarure gauge?

Hi.
I have a Fiat punto ELXJTDi and for some reason the temperature gauge has gained a mind of its own. When the ignition is switched off it does not return to cold. Sometimes reads 1/4 way up the gauge or 3/4 or half way. This causes problems when driving cause it reads overheating when it isnt. The water levels are fine apart from the radiator never seems to get really warm when idling.. Is it a sensor ? Is it an airlock.. Head gasket replaced last year but this problem has only been this week. Please help
Paul