How stiff is the wing in torsion? It might not appear twisted on the gound but at speed one wing might be twisting more than the other due to the pitching moment of the wing section.
I have an electric powered glider that shows the problem even though it's got a carbon spar, a kevlar D box leading edge (layed up at 45 degrees) and carbon capped ribs. It's a great flyer but if you get it going a bit too fast it will roll and tuck on you. It's allways the same direction - probably one wing is slightly stiffer than the other.