Thanks again. Will look into it. I *thought* ntpd was careful to avoid large changes in updating the clock, but maybe not careful enough.
Update: Ubuntu docs:
Hm. Define "large" ...The ntp daemon ntpd calculates the drift of your system clock and cont +inuously adjusts it, so there are no large corrections that could lea +d to inconsistent logs for instance.
The way forward always starts with a minimal test.