I'm not expert but it sounds like there's some binding when any movement is made to the rotor's orientation. That being the case, check the swashplate for any binding or other issues since it's the primary part involved in that process, then work your way up. I'd disconnect the servos and make sure all the movements are smooth and easy, including pitch changes. It's probably something tiny and minor, but it doesn't take much with this things I'm finding.