I just got an email from someone who had a similiar experienc. They felt that the Reversing Y was picking up some RF and that is why it only affected one side of the Y. The Y is not marked as to which is the Reversed lead, but I can connect each flap servo, one at a time, to the Receiver, and see which side is reversed and whether the Y is part of the problem.

It rained all day today, so I could not put the plane together and test it.