So, going under the assumption that the problem started with the chain-change, I spent some time thinking about what could cause this about a new chain (keeping in mind that changing the rear cassette did not help).
I thought that maybe the quick links could be a problem, perhaps snagging on an adjacent gear when going through the cassette. I examined them, and they clearly were thicker/wider than the adjacent chain links.
So I took them off, and they were labeled "8 speed." I have no idea why 8-speed links came with a 9-speed chain.
So I went out and bought a "9 speed only" set of quick links, and installed them; these were the same width as the adjacent links. Then we went out for a couple of miles and the problem did not occur (it has never gone more than a few hundred yards without occuring, since it started).
We won't have time for a longer ride until tomorrow, but I'm pretty confident that this has solved it; I'll re-post only if it didn't.
Thanks for your help.
Mark
Last edited by 124Spider; 09-02-19 at 05:11 PM.