I can't see how air can cause this, air if any would accumalate in the radiators and they would not get hot at the top.
I would suspect the high flow of water across an oriface, you know how they pump out ships bilges by this method, it creates a vacuum.
Maybe your pump needs to be on a slower speed, or maybe the lockshield valve is barely open.
I can only suggest you try altering these to see if there is any change.
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