It could be one of a few things.
1. Flex. See if you can see where it is flexing.
2. Uneven movement. If the pushrod is way off center where it attaches to the servo, it could cause one elevator to move more than another (so at center, everything is fine, but at each extreme, you'll see one elevator move more than the other)
3. You simply have too much throw and you're stalling the plane.

Hope this helps.

-Mike