Had another "rassle" with this today, but no joy

I looked through the "3-point shudder" links and they gave me some ideas of things to check.
First thing today was to see if I could physically lift the PHD when attached normally (ie bracket and lift arms.) Not being the 220-lb. linebacker I never was nor related to Arnie S, this was a challenge but I figured if I removed the auger and if necessary the gearbox I might be able to do it. Got the auger removed but not the gearbox as I didn't have the PHD suspended again after initially checking that the problem was still extant; couldn't hold it properly to knock the pin out. Nonetheless I was just able to ascertain that the PHD would lift without obvious binding.
Then tried detatching the boom where it is pinned to the bracket and holding it loosely with some NMD14 wire as I couldn't find any suitable rope. Unfortunately the jerking when lifting recurred, so I am fairly sure that geometry isn't the issue.
I did check the movement of the FB rod as well as the connection opposite the Position control lever to be sure the roll pin was intact etc. and everything seemed normal.
I then rermoved the PHD entirely and checked the lift vs the movement of the Position Control Lever. When I just push the PCL back in one movement, the lift arms rise smoothly., as in the video I linked earlier. But when I push it back incrementally, ie to see where the lift action begins, it does seem to lift in stages, although smoothly. (???)
I now have a sore back, so no morre rasslin' with the PHD for now. Tomorrow I'm going to put the brush cutterr on and attack my field. But I suspect that I will have to get the local dealer to look at this.
Addendum: Also tried running engine at higher rpm- around 1800.