Basically, it's like the frame-rate drops to around 50% for a second or more and then returns to normal; completely randomly. But, oddly, it's replicacatable; as in, if I 'skip back' a few seconds it happens again in the same spot. (I don't watch anything "live"; only ever recorded.)
If I force my Tivo to only output HD (ie 1080p) then the problem goes away and the picture is smooth.
Not sure if this indicates an issue with the output from Tivo @ UHD (or, at least, upscaling to that) or the TV.