RE: Duellist maiden- with Jett FIRE 50's and pipes
ouch.......
not sure on the porpousing ...... usually something like that come from a 'too sharp' leading edge on the wing - where the plane wants to hunt up and down a bit. But I do not recall seeing that being the case in your photos.
Other than that... the obvious stuff.... like a servo that did not want to properly center, or a linkage that "buzzed" and flexed a tiny bit at a certain power setting.
Bob