Could it be? Assymetric thrust in a model size? It sure sounds like it. The prop on the right hand side has more Angle of Attack as it sweeps down then the left side has. As a result it pulls forward more on that side until the tail comes up and the prop disc is more in line with the direction of travel. Common handling problem on the warbirds but I didn't think we'd see it on a model.

How do the real planes handle it? More rudder throw I'd think?