One of the best tools available in IIS 7+ to debug the applications and find out about the issues happening historically on the web server is FREB log files. FREB as it stands for Failed Request Event Buffering, is a handy tool for tracing the status of an application. It basically hooks up to an http status code and persist the tracing information in an xml file. While the xml file is not a small one, there is a very useful transformation file that renders the file in a readable html format (it only needs to be placed in the same path of xml file when it is being opened by your browser).
FREB can be enabled using web.config tracing element [here], or simple using IIS7+ admin console.
During the monthly testing, I had to extract few thousands of log files and check what are the main issues with the web server and fix it. I wrote a very simple application that helps me automate the process by extracting, filtering, and persisting data to the database.
You can get the source code of this application at: https://github.com/wizact/FrebParser
Please note that this application is provided without any guaranty to work as you may like.
Cheers..