Hmm. Couple of suggestions if you're not getting the information you need to trace the problem:

Use ProcMon to monitor file activity.
Eg: Filter on a path or 'Category is Write'

As a stop-gap you could use SQLServer to manage session state (configurable in 'Session State' settings for the WebSite)

Also (I think) IIS7 allows you to monitor recycle events (Advanced Settings on AppPool / Generate Recycle Event Logs) which would make your code redundant.