I have NO direct experience with this myself. However, I can think of a couple of ideas.

My first thought was that your rudder was having problems with the servos being overloaded or with flutter when you use the rudder in that extreme of a manner, and maybe you should increase servo torque and stiffen the linkage and maybe the rudder itself.

Your question about the triangle stock reminded me of something I saw the other day on a 40% Extra. The pilot had slit fuel tubing lenghtwise and taped it to the TE of the rudder. This apparently fixed a tail waggle problem in (if I recall) downlines. He didn't have an explination of why it worked, just that it did work and it had been suggested to him to try. Looked funny, but worked well. Since t's easy to try and undo, you might try it.