What happens while in a turn? Does it tend to hang with the tail low in a steeply banked turn? If you use only ailerons to roll into a turn does the model show signs of adverse yaw? Or does it do the opposite and wants to tighten up all but the shallowest of turns?

I'm betting there's some adverse yaw from aileron only inputs and that it runs around a steep banked turn with the tail hanging low and the engine pointed to the "top" of the circle cone of the turn. That's a sign of too small a vertical tail and the fishtailing you're seeing is a slight bit of dutch roll. It gets worse when you're climbing, right?