I have problems with an application pool on windows server 2003 R2, IIS 6.0. Sometimes the application pool stops for unknown reasons. I would like to research this. What could be the reason? Where could the stop event be logged?
First place to look is the System (and possibly Application if your app logs "help me" type events) Event Log.
If an Application Pool worker process (W3WP) crashes (by default) 5 times in 5 minutes, it'll be marked Stopped by W3ADM/W3SVC.
This feature is called Rapid Fail Protection, and it's designed to prevent bad applications from using up all server resources by crashing all the time.
It's configurable in the App Pool properties.
If it's crashing (or exiting, or you're just curious), get DebugDiag 1.2, install it, and create a Crash rule that monitors the Application Pool you're having a problem with.
Commen error is that the web application crashes a few times, and after this happens X times, IIS will stop the app pool