The transition from a flat jumper to a round, active, thrusty jumper is very difficult, and a horse with a round powerful jump will really highlight any and all position flaws, both design flaws and mechanical flaws.
Your overall *design* of position is correct; the problem is with the mechanics of your position. You're used to being in a fairly static two point and just maintaining position over the fence; you new horse's push and thrust requires that your joints act as shock absorbers and allow you to stay with his motion.
I also suspect that your old horse was fairly quick in the air; and your new horse has "hang time", maybe even dwells in the air, as one of the problems seems to be snapping back or unfolding early. So you muscle memory is to open you hip angle and sit up at a certain time, and that timing is too early on the new horse.
The solution is pretty much what the other posters have proposed. Take a few steps back. Work primarily in grids and gymnastics where you can really focus on position, both the design and mechanics. Don't attempt difficult turns, courses or tricky technical distances until you're more comfortable staying with his jump. Very low grids without stirrups, approached sitting, focusing on waiting for his thrust to close your hip angle are a good idea, as well as gymnastics that allow you to really concentrate on allowing your body to follow his motion while staying off his back through the entire sequence.
Also focus on staying out of the tack over the fence a beat longer than you think necessary as a short term fix. Once your mechanics are correct and you're staying with him better, this may not be necessary. What's happening now is that his thrust is acting on your stiff or locked hip or knee, and it propels you forward out of the tack. Every action has an equal and opposite reaction, so you snap back with equal abruptness and force.
If possible, try to ride or take lessons on some horses that are somewhere in the middle between the style of the old and new. It's hard to be soft, relaxed and focused on body mechanics when you're worried about finding the saddle by radar on the other side of the fence because you've been jumped out of the tack. (Believe me, I know that feeling!)
I really, really like your new horse, and he will really broaden and deepen your riding skills. Good Luck, and I would love to hear more about your progress!
Last edited by maura; 09-03-2011 at 12:04 PM.