I can almost guarantee that you are simply giving it too much elevator. I had this same problem with my flying wing and it was snap rolling at the top every single time until I learned that I had to fly it through the loop and give it very little elevator. Now it does loops as it should and there is no more snap roll problem.

So, I would recommend that you take it up to altirude and see what happens when you actually fly "through" the loop, instead of just simply holding max elevator.