Random asp.net Session Timeouts

Random asp.net Session Timeouts

Need some ideas on random asp.net Session Timeouts. Environment: - ASP.NET
3.5 Intranet Web App using Windows Authentication. - 1 Web Server - Web
Garden Configuration (multiple worker processes). There are multiple
automatic recycles done throughout the day based on memory limits set in
IIS for the app pool.
- Session State Mode : StateServer (using the asp.net State Service
running on the web server). THIS IS OUT OF PROCESS. - Session Timeout set
to 480 minutes in web.config. Out of 400+ users, there's seems to be a
small % that will frequently lose their session due to timeout. This
happens randomly throughout the day.
I'm pretty sure it's not due to the recycling. I can do an IISReset and
still keep working with the same session since the session state is kept
in the stateServer. As long as I don't restart that service, the session
sticks around.
One thing that came to mind is maybe the session cookie is not making it
to the web server, but not sure what would cause that scenario.
Been racking my brains on this. The configuration I have should allow
active sessions for 8 hours.
thanks in advance