Nov 14, 2017 07:38 PM|SharpJoe|LINK
I realize this is a really old thread but as we had this exact symptom and finally ran it down I thought I'd share.
We had some background processing that is triggered and runs in a thread from ThreadPool that had an issue. The part that may be a "well duh" to some of you but wasn't obvious to us is that the code running in that background thread (a report / computation
function waiting on another server to prepare stuff in our case) is NOT covered by the Global support from ASP.Net/HttpApplication like the Application_Error handler and as such has to be defensively coded.
If that code throws an uncaught exception it will not go to your application error handler AND it will take IIS down in exactly the way described in this post. Adding a catch for the exception (or a catch all if appropriate) fixed this issue for us.