Summary: how to diagnose and eliminate the xmlrpc.php WordPress exploit when it's clobbering your web server.
You've been running a public Ubuntu 12+ and Apache/LAMP web server for a while now. Until recently everything has worked fine, but suddenly your server is struggling to display even static pages. Sometimes content takes forever to load, and occasionally pages fail to load at all, with Apache showing the 503/Service Unavailable error. You haven't made any recent changes to your applications, server software, or databases, so what's going on?