I would double check that the floor is properly assembled and that the keel is inflated to the correct pressure. My first guess from the symptoms is that the bottom of the boat may be deforming slightly as you speed up, putting the prop in disturbed water so it ventilates. If you look over the transom do you notice any visible change when the problem occurs?
By the way, according to the Seapro importers, the 470HD is only rated to 40hp:
http://www.seaproboat.co.uk/inflatables.php
Not necessarily relevant, but might be!