Perhaps if you take an extended run with the heater on full you could ensure there is at least no air trapped in the heater core? Does the heater put out plenty of hot air in the cabin by 'ram effect' at speed without the (interior) fan being switched on?
Do you observe the colored coolant liquid in the expansion tank when the engine is cold? Mine has a metal tab indicating the 'fill level', when I did the flush and refill I couldn't see the liquid level a few times without topping up.
