Hmmm...I'm going to assume that the wing was built straight, and that you have no aileron servo movement when you move the elevator...Is the elevator level? That is, is the elevator in the same plane (geometrical...hehe) as the wing?

I'll also assume that you are talking about a problem that occurs at normal flying speed, and that your elevator throws are not excessive. Excessive elevator throws can cause an otherwise normal airplane to snap at most any speed...

How dramatic is the turn? Are you snapping? Or is the plane making a gradual turn?