I believe I have found the solution to this myself, by reading the words of the wise Mr Graham Dumpleton, author of mod_wsgi: "if you are going to leave these running permanently, ensure you use --server-root option to specify where to place generated files. Don't use the default under /tmp as some operating systems run cron jobs that remove old files under /tmp, which can screw up things."

I was running things under /tmp. I am now adding --server-root to my "python manage.py runmodwsgi" command, and will see whether this resolves the issue.