I had a mid 80's 19' Howard Sport Jet. Powered by Gale Banks twin turbo BBC. Same problem and this thing had plenty of power. Just my opinion, but I think it's a hook on the bottom of the boat. Also my bro-in-law has a 95' 21' Ultra jet with open bow. Stock 454 power and same problem too. Even at regular cruising speed on glass smooth water it porpoises. Just my opinion, but definitely think it's a hook in the hull.
Steve-O