It just turned cold around here, and as of yesterday, the headrests are working perfectly. No delays, no annoying "clicking", no taking turns "maybe" going up.
They both went up and down in unison, every time.
The RPMs didn't seem to matter, but it WAS cold (mid 40s), so maybe there IS something to your theory, Mike. It just seems that they should work better in warmer weather, not cold.
Oh well, more "study" is required here.