Zodiac Bayrunner 420 RIB w/Yamaha 40 porposing

[Theo]

I just launched my new Bayrunner w/40 hp Yamaha 4 stroke yesterday. It had been setup and had sea trial from the dealer. We got out in the backriver channel and brought the boat up on plane. As soon as it began to plane, the boat began to porpoise. It didn't take long, a matter of seconds before it was out of control. No way could you keep that up. The bow was undulating up to 2 feet up and down. Not sure how far it would have gone if I kept it up. Tried power trim but increasing the trim i.e. moving the motor away from the transom only made it worse. Anyone got any ideas. With motor trimmed down to transom stops. The cavitation plate projects to be 3/4" above the edge of keel. Then cavitation plate is at approx 10 deg angle to the keel. The cav plate is at a positive angle to the keel. Projecting the keel back to the prop the prop appears to be about 3 inches below the keel. see pics The dowel is the keel projection relative to the motor. anyone have this experience or any ideas?