Gently tap a table sideways and a unipivot will behave exactly as Steve describes.
Didn't the KML have a problem with the Mercury corroding the 'baths' over a period of time
The more I think about this the more I suspect it is structural (floor/table) related. It is the only thing I can think of that would clearly impact a mass below the pivot point in the way described.
It could just as easily be footfall, passing trucks etc. A suspended wood floor is essentially a diaphragm/soundboard. It moves. A turntable stand, especially if mass-loaded (e.g. big heavy amps etc) can amplify the movement or lower it in frequency. This is exactly the sort of thing a unipivot will react to as it’s mass is suspended. Gently tap a table sideways and a unipivot will behave exactly as Steve describes.