Hi, I am getting exactly the same problem just as randomly - having spent weeks running through the code, the only logical answer is that the session is being terminated prematurely.
I moved from my last host (that had the problem), to a new host (that also has the problem) - they are both Windows 2003 sites and don't support Windows 2000 servers.
Can anyone help with more information on why the session operates differently on Win2003 servers, or an alternative way of keeping track of users and last visit dates ?