I take it that you have new blades on ?
Are the blades balanced length and chordwise? Any blade twist will change tracking with changing headspeed.
Is your swash ok, tight with no play?
Your main rotor housing could be bent, but really it would effect tracking through the whole RPM band. I would check every ball link on the swash, every head component for play, basically from the servo's to the rotorhead.
That's my best guess