might be an air bubble around where the coolant goes into the IACV on the throttle body, therefore not allowing the IACV to know the right coolant temp and moderate the idle properly. In any case all you can do really is run it until its hot, let it cool, and repeat over the next few days if it keeps doing it. Theres no real problem in having a few little bubbles in there, just watch your temp gauge when you drive as if there is a large bubble it will overheat