I have an issue when randomly Outlook 2007 SP2 clients get prompted to authenticate after being logged in for a certain (completely random) amount of time. Exchange and Outlook have been up and running for over 2 years without issue, this issue just sort of cropped up recently so I thought it may be related to a WIndows Update. I have over 5000 clients, and a very few are reporting the issue, but I want to get to the bottom of it. When you check "connection status" sometimes it shows "directory services" as disconnected, sometimes not. We have multiple DC's in each AD site, no AD issues, no DNS issues. It's not an autodiscover issue. YOu can run the "test email autoconfiguration" and it passes just fine. Restarting Outlook also fixes the issue 100% of the time, only for it to re-appear maybe an hour later, maybe 3 hours later, maybe 3 days later.
Backend is Exchange 2007 Enterprise SP1 UR9 running on Windows 2003 R2 SP2 x64 Enterprise. Have separate CAS+HT servers (2 in an NLB) and Mailbox Servers (4 servers in an SCC config 3 active 1 passive).
Clients are all XP SP2, Outlook 2007 SP2, all running in cached mode.