i had this issue, where the plane banks uncontrolably to one side and then spirals to the ground, though for me it was’t unik to the Arrow, had this with the X-Cup and TBM. sometimes it helped to counter the movment while the AP is on 'till it switches off and repeat this until it stays level again. Other then that, sadly i have no clue.
i just found that i can provoke it to malfunktion when AP is on and you go to slew-mode (Y) and back, it stats acting up.