If it pitches up like that, especially at low speeds, the CG is too far aft for the geometry of the plane. As the nose pitches up a little, the wing's Center of Pressure moves forward, causing the nose to pitch up even more. Move the CG forward a little at at a time until it flies the way you want it to.

If it only happens when the prop is generating thrust, then look at the thrust angle, but I don't think that is the problem, based on your description.