I think that I would have to agree with Charlie P. It sounds like something is giving way in your control setup. The most likely culprit would be that the control line/rod is flexing. This means that when you move the elevator surface the rod will flex and allow the elevator to move back to center briefly. But you should also look at your elevator servo. Move the servo to full extension and push on the servo arm and see if it moves back any.

This would be my opinion on what you problem is.

Ken