I think Doc's description of the symtoms may be causing confusion. As Fast Fred says from the descripion it does sound as though the boat is ploughing and attempting to hook a bit but, as Wavelength says, one would expect the 4 stroke engine to be heavier and the CG to move rearwards which would tend towards chine walking instability. And as Wavelength says, one wouldn't expect a bit of movement of the CG to affect the boat in a way which couldn't be trimmed out. However, since we are talking about a Humber Destroyer, I'll relate a wee story. My previous boat was a 6.5m Destroyer and we owned it for a good number of years. I felt it was a good handling boat and it was stable at 50mph but it did need fine control at that speed. Anyway, it was sold to a group of divers and it was arranged that we would launch, go a trip and recover so that they could get the hang of the system. The boat was normally set up to carry 2 persons and the seating was towards the rear although the fuel was forward of the consol and a pile of chain and an anchor was kept in the bow locker. So off we go, myself driving and my wife sitting beside me and 3 big divers standing behind and holding on, half a tank of fuel and no anchor up front. Well, it handled like a bu**er, chine walking at anything approaching 40mph. Trimming down brought back some stability but nothing I did made me confident once the speed increased.
Plainly it was possible to tip the balance of what was a good handling boat by making it stern heavy.
I wonder whether this and Doc's lack of experience could be contributing to the instability he is experiencing, especially if the boat is light and carrying little equipment.