RE: Need help with terrible flying airplane
Look for an experienced flyer who knows how to eyeball an airplane for "straightness".
He will sight from the rear and look for twists and wiggles. He will also give you some hints on how to straighten what isn't right. It's almost always fairly easy to do. Using an iron to straighten out warps is easier than you think, and the Decathlons I've had all needed some of that.
He'll also look for unbalanced surface deflections. And engine thrust problems.