Well joy-joy!   Appears you were both right …I think.

Poked into the bowels of the file and found an unusual couple of lines of code in the .pz3 referring to “layer kfrm”, with the offending Hip:Y-Trans value set to Constant.  (was all set to Spline in the normal Keyframe window).

Now, I have absolutely no idea what an “animation layer” actually is – does that function even exist in P6?  --  I did do a quick RTFM, but didn’t see the term mentioned.  Will have to see if the good Dr. has a tute.

But, anyhoo, I whacked that part of code from the .pz3, & it’s working like a champ again. 

Thanks to you both for the cooperative head-scratching.  New turf for me & it was driving me crazy(er).