Maybe I missed it in your post, but I didn't see a loader in your list. If not from what I've newly learned myself, you need one with that kind of speed and horsepower. regardless if it helped the porpoising problem or not.
I guess the obvious question is: since you have a place diverter, what inputs do you put into the diverter to stop the porpoise at WFO? I would think that would give you an idea about what is causing the porpoise. I'm assuming that when you drop the PD down a notch or two it goes away right?
Nick