Does the wing have shear webs between the top and bottom spars? If not, I have fixed this problem in the past by holding the wing and twisting it the warp out and then having a buddy super glue a shear web across the spars between several of the ribs and the twist will not/can not come back. This also makes the wing virtually twist proof from then on. The web is made from 1/16" balsa as wide as a bay and the grain running from spar to spar. Good luck.