I don't have the Fly zone cub, but I have several Parkzone and Hobbyzone cubs. When one of them does this it is usually either the vertical stab flexing, the entire tail section flexing, or the elevator flexing. I have had a warped wing on the Parkzone J-3 cause this. I also had a loose strut on the Hobbyzone cause this once, but in this case it was a very gentle roll. I would look closer at that tail section.