It's true that a "snap" implies a stall. If you're sort of barrel rolling to the same direction every time you pull up (which looks like a fast, sloppy snap roll), uneven elevator deflection seems really likely, particularly where you have laterally balanced it. Is your linkage such that it applies force to the right half of the elevator more than the left half? That is, would the airflow tend to twist the left half toward neutral more than it would the right half when you pull up? I've seen a sport arf with this problem before.

I would guees that a warped wing could do this too.