Sounds to me like you don't have the airplane set up well, and you're expecting it to fly like it won't. Here's the general rules for 3d type airplanes, build it light, and fly it right. If you are at high speeds, do not pull huge amounts of elevator at high rates. Half throw at half throttle should make the plane snap, if it didn't something would break, as you are imparting too high of G-loading for the airframe. If you are flying around at higher speeds, it needs to be at low rates, or with high expos on high rates. Don't use the large throws except when flying slow 3d/high alpha type maneuvers.

Things to look for on this plane to cure snapping problems, wingrock in high alpha, weird rolling tendencies etc are CG (front and back, and side to side), even control throws on each half of the elevator, and having the elevator halves matched as far as speed. If those halves have any difference, it'll roll with elevator input, which combined with a high speed stall (like what you are getting into) will make it snap.

As far as your power problem, I ran the same plane with the same engine with an APC 17x4W prop, and it flew beautifully. I kept it light with a small 4 cell pack, and mini servo on throttle, and it hovered at about half stick, with plenty of pull out. Incredible flying plane all around, did the nicest high alpha knife edge you've ever seen, and was very stable in the hover and torque roll.