The forum was slow to load and we received the following error message:

"The following tasks appear to be locking frequently: clearsessions"

Shortly afterwards, the site was inaccessible until we repaired the crashed core_sessions table in the database.

The forum is now running normally, but I'd like to know if this is likely to re-occur, and if so, how to avoid it please?

If your database tables are not using InnoDB, I would encourage you to verify with your hosting provider that they are configured to run InnoDB tables in MySQL optimally and then enlist their assistance in converting your database tables to InnoDB.