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After resetting permalinks, it takes up to a day before the problem recurs? If you’ve deactivated all plugins and the problem recurs with no active plugins, the cause is unlikely to be a plugin, unless it’s a “must-use” plugin. If you have any, they are in /wp-content/mu-plugins/. They can be deactivated by renaming /mu-plugins/ via FTP or your hosting file manager.

It’s possible your theme is the cause. Try using a default Twenty* theme for a while to see if this resolves the problem. It’s difficult to debug this kind of problem, one that can take a while to manifest itself. Ideally you’d identify some kind of action that reliably causes the problem to occur, such as requesting a particular page.

While rare, it’s conceivable that core WP had become corrupted. You might try performing a manual update, even if it’s to the same version. The point is to freshen all the files from a fresh download. The Re-install version x.x.x button on the updates admin screen does not do this reliably

Thanks, that was useful info. I’d done some of the things you suggested.

The problem now seems ro have been resolved. I had a chat with my hosting service and having been directed to check the site logs for errors in CPanel, also discovered they had been “throttling” the site, confirming a suggestion server syncing was, potentially, the issue. I’ve reinstalled a caching service and I will also look at the theme – it hasn’t been updated for ages by the developer, and I know the menus are overloaded, impacting mobile presentation, which is something I need to fix.