If the extra has split elevator servo's, check to see that both elevators travel at the same speed and travel the same distance. If there is any difference in their travel , depending on how much, they can act like ailerons. If the elevator is single servo with a connecting rod between them, check for twisted or warped surfaces. If neither is the case then you need to probably reduce the throw on the elevator. Some planes tend to roll with too much elevator applied. Hope this helps.