Shattered plastic coolant impellers are the first thing to come to my mind. Also, how well do your electric cooling fans work? Sometimes the cooling fans use a different sensor than the ones used for the gauges and/or the ECU. Do your cooling fan relays work?
Sometimes the fans activate only when the AC is on, if the cooling system does not function properly.
Sometimes the fans activate only when the AC is on, if the cooling system does not function properly.