Uncle Sir,
i had a bit of this prob before,....it was due to 2 factors.
1st when i broke blade grips I didnt notice that the part where the mixing arms attach to the blade grips had a little shim and a washer (cuz they flew off in crash)
So I assembled it with out those and tightened the little screw that holds the mixing arms onto the blade grips too tight and the mixing arms did not move freely enough.
The second part of my prob was a lack of lube EVERYWHERE. Make sure the spindle and Orings are lightly lubed and the Center hub/rotor head and the mainshaft where swash sits. everywhere (except ball links) that moves when pitch is increased/decreased. detach servos from swash and then do some playin around by hand to confirm that everything moves very smooth and freely.
If you have already tried all this just disreguard my ramblings.
I just know it was like magic when i figured all this out on my bcpp. This also got rid of a wobble in the bird while hovering that I had been going crazy over for days...