IIS 7.5 with Error 503

Windows 2008 R2 (Amazon EC2) single-site web server that, since the start of this week, suddenly started to (randomly) stop serving a PHP (Joomla!) website with a 503 error page (a blank page for most of the browsers). I've tried to investigate logs and to modify some IIS settings with no real (definitive) results (every few hours I have to restart IIS or the whole Windows server). Sometimes IIS goes down (503) and immediatly after comes up without any intervention by me. My best guess , based on logs (also httperr log files), is that something is going wrong with fast cgi ([login to view URL], php version is 5.3.24) and this push the Application Pool of that website to not respond anymore to IIS (I repeat , this is essentially a single website server). Minimal results, too, disabling Rapid Fail Protection option. Very disappointing. I'm not able to definitvely understand why this is happening, without any apparent reason (no "big" installations or new configurations before the moment in which IIS started to put 503 errors) and I need an help to understand the problem and to fix asap.