And some model aircraft (and full scale as well) exhibit pitch/roll coupling inherently, and there is not much to be done about it. But, I would add quite a bit of expotential to the ailerons and the elevator. He's right, you may have way too much elevator throw, causing the model to try to, "Snap" with full elevator. Try adding the expotential to soften the input around neutral, and try to lower the endpoints or servo travel.

Bill, AMA 4720