I'm no engineer, but my first impression was that you're a little overpowered on a .40 size model using a .61 size engine. This being the case, when you yank back hard, your airspeed slows and engine torque may take over, causing the snap roll. Does your ship snap opposite to the rotation of the engine, if so this may be possible. Just a thought, but at least (although for different reasons) I agree with the thread about reducing your engine size.