PHP IIS App Pool and URL Rewrite


We have a php application running on Windows 2012 R2 and IIS 8.5.9600.16384. The php application uses CodeIgniter as the MVC framework. We are also using the IIS URL Rewrite module to remove the "index.php" from the url.

Over the past couple of months we have noticed that at apparently random times the URL Rewrite module will stop working thus causing a 404 error page to appear because the url does not contain index.php as designed. Simply recycling the app pool corrects the issue but we are having to monitor this all the time. We have adjusted multiple settings on the app pool in attempts to correct the issue without success. We have the exact same url rewrite rules in place on other applications and they do not have the same issue which leads us to "it is something in the code of the php app" but we are struggling with any code that would cause the app to stop processing the url rewrite. If we do not recycle the app pool and manually add in the "index.php" in the url