WLM
What you have described is a Chordwise mass discrepancy on your main rotor.
The twist of the blades as a function of power (blade coning) is controlled by chordwise mounted tip weights (tracking weights) if they are at odds you can only tune the rotor to be good for a narrow AUM range using pitch link and tab corrections.
If you want to cure it you may have to get the blades reweighed and checked for spanwise and especially chordwise CofG , there will be a tolerance on the figures and I suspect you will have two blades on opposite ends of the tolerance. Has one of your blades been repaired I wonder?
Here's a post I did earlier describing how the effect is used to control the track and vibes on a Bell 412
http://www.pprune.org/forums/showthr...19#post1560419
Cheers
W