I think my Webapplication gets shut down after a while.
It returns a new session if I haven't used the application in maybe 5 minutes. The session timeout is set to 720 minutes so that can't be the issue.
Is it maybe a setting in the Application Pool or something like that? I figure it is some sort of resource management. I use IIS 7.0
We had the same issue with a web service that had to stay online in such a way that we couldn't afford the latency involved in starting it up if it went stale.
We wrote a very simple Windows Service that woke up every 3 minutes and requested a page as @Wayne suggested. Worked like a charm.