overheating after flushing radiator and changing thermostat

Hello, what was checked to determine the radiator and thermostat were suspect and needed to be replaced? Also, have you ensured all air is purged out of the cooling system? Air that remains can create an overheating situation. Lastly, when does the overheating occur? You say "sitting still" but not sure if that's when the overheating occurs or when it's running OK?

Make certain radiator cooling fans are operating properly.