From reading this thread, it sounds to me like you hit your peak resources. Something you should in in the future is run an add-on like:
Developed by LiteSpeed Technologies - the LiteSpeed Cache Plugin for XenForo 2 (LSCXF2) is a PHP-based plugin that communicates with your installation of LiteSpeed Web Server(LSWS) and its built-in page cache, LSCache. Because LSCache is built...
xenforo.com
It would help if you got overloaded with guests like you mentioned. I believe
@MattW would be able to help you set this up if you switch hosting to his.