I thought I was getting somewhere as the errors seem to occur roughly every 15 minutes, which coincides with the cron job:
*/15 * * * * wget -q -O – https://www.mysite.com/wp-cron.php?doing_wp_cron > /dev/null 2>&1
I turned off the only job that WP Crontrol said was due:
Abandoned Cart Lite for WooCommerce (the free version on wordpress.org/plugins)
but that didn’t seem to solve it, even turning off the plugin altogether.
Turning off the cron job altogether certainly seemed to reduce the occurrence of problems but not eliminate it.
Once again the only sure way of eliminating the problem was deactivating this plugin.