Engine heating only when idling or when driving in town with red lights

Fan cuts on and thermostat replaced. Possible air pocket in cooling system. Water pump driven by timing belt. Lost antifreeze a while back and might've got an air pocket. How do I get air pocket out. Stays cool on highway or in town with no stop and start a lot. Just heats up when not moving like in drive thru line and bank or fast food place, or numerous red lights.