I have the exact same problem on my Lanier 1/4 scale Extra 300. I only got about 3/8" throw on the elevator and same thing, pull a little to hard in making a loop and it'll snap. It seams when you pull hard into a loop it snaps when it gets to about 45* and if you hold it there it will turn into a roll. Strange.
Does wing loading effect how quickly it snaps? A guy at the field figures it weighs about 15# and he said on a 72" ws it was too much and causing it to snap.