Assuming there are no mechanical linkage problems, what would cause a plane flying in high alpha knife edge to rock up and down. Acts as though the rudder is stalling... recovering and so on. Condition worsens with greater rudder deflection. The plane in question has a huge rudder to fin ratio, long tail moment.

I have noticed full scale aerobatics planes with triangle stock attached to the trailing edge of the rudder. Whats the purpose of this?

Kevin