I’ve disabled all plugins and the site works fine. I enable the The Events Calendar and The Events Calendar Pro plugins and the site slider and menu system fail to function properly. I enable jQuery Migrate and jQuery Updater and the problem persists.
TemplateMonster.com will not provide support as they’ve ruled out it’s not their theme at fault, it’s a plugin, specifically The Events Calendar and The Events Calendar Pro. Note that before we installed the Pro version, all was well and everything worked properly. It was when we installed The Events Calendar Pro that the site became unstable.