Have you crashed it yet? if so then your feathering shaft may be bent. take your head apart. check the o-rings and the bearings. Make sure your thrust bearings are in the correct way.

I just fixed my heli that had a similar problem. i couldnt figure out what it was but i knew it was the head. tracking was always off no matter how hard i tried. took apart my head and changed the feathering shaft, o-rings, thrust bearings, and bearings and she is good as new.