Well... I've read all the CX posts on this board and don't recall on like yours. All I can recommend is to go through it thouroughly. Make sure your shafts aren't bent, collars in the right place - not unnecessary binding or play in the shafts. Look at the meshing of the main gears and see if there's some - just a little - daylight between the pinion teeth and the main gear gullet. Maybe there was stress and a bounce released it so that the pinion gear is now too tight against the main gear? After all that, I'd be tempted to turn down the gain pot and crank up the proportional pot to see if you can get the blades back in synch. I try to get the proportional pot dead on with no yaw (with the trim centered) then bring the gain back up. Hope that helped.