Thanks for the update, @pulsebaynz.
If you couldn't identify any of the plugins as the cause of the issue, I'm afraid there is not so much we can do.
In case it is useful for you, you can use WP-CLI utility to do some analysis. In a command line, access to the server and execute:
wp transient list --fields=name,expiration
You can delete those transients that expired:
wp transient delete --expired
You can also automate the deletion of the transients with cron. Edit the cron entries with:
And add the following line:
@daily wp transient delete --expired
That configuration will execute the command in a daily basis.