are both blades balanced the same? also are both blades tracking the same? another words one blade is not tracking high and the other one tracking low?
any wind when you were flying? sometimes that can cause it to do that. i know on a real helicopter it does that so i would think on a model it would be similar.
also has the helicopter been wrecked before? is so make sure nothing is bent like main mast,feathering shaft,flybar and make sure the paddles are the same distance.