An air pocket generally gets trapped in the intake manifold and stays there giving you a false "cool" reading 'cause the engine temp probe will be sitting high and dry.

Check the tension/condition of your alternator belt. It drives the water pump too. I had an oddball situation when the bolt that holds the tensioning bracket broke so the alternator just sort of bounced on the belt. When I drove there was enough air through the radiator to cool it down, but at idle it would overheat. It could be the water pump too but if it's not leaking and the shaft isn't wobbly it's probably ok.