Well you now know that porpoising is not caused by just one thing. It can be a combo of things. They first should be making sure the hull is blueprint flat in the planing area. Any convex or concave can cause porpoising. You should be able to lay a stright edge on the hull (foreward and aft) and it should be dead straight.
Check your weight distribution, check your set back, check the height of the motor on the transom. Do you have a jack plate? Can you raise and lower the engine? A study done by Mercury (I think it was mercury, sorry if I got it wrong) showed that a lot of porpoising is due to slack in the steering system. There should be no slack at all. I honestly don't think it is your prop, but it could be. If all else fails try that.
Back to weight distribution. Where is your fuel tank? If it is low in the boat, amidships or near the stern it's probably ok, but if you have a bow tank it might not be. As you use gas the tank weighs less and changes your center of gravity which changes the center of force on the hull when at speed.
ANyway. keep trying untill you hit it.
Oh yeah I forgot trim. Trim can have a serious effect on porpoising. Try various trim angles.