Are the elevators moving exactly the same amount and at the same time

Easiest way to check this is tape a peice of straight balsa stick to the top of both elevators and point them backwards to meet right behind the rudder and slowly move the stick to apply elevator and the sticks should stay lined up all the way through the travel. Then try it moving the elevator quickly, this will show if one servo is slower than the other

By your description, I'm guessing the left elevator is either moving more or moving more quickly.

Oh yea. The sticks should line up right from the start as soon as you tape them to the elevator

I've also seen guys glue the sticks to clothes pins and click them on the elevators