We have some progress. I woke up in an armchair last night and checked and wsjtx had done its stopping thing and the terminal window it was started from has a clue:
[20200122 11:57:54.238 GMT D] D-Bus global menu: no
Assertion 're->data || re->memblock' failed at pulsecore/pstream.c:862, function do_read(). Aborting.
Something to do with a memory block failure. Is this a bit of bad memory on the Pi's wee memory card? They are notorious for having defective blocks.
If so, can I run some utility to scan the card and rule out sending data to bad blocks?