Ok - still happening. Very random. Will post exact error message from error logs shortly.
Here's the info from the Event Logs:
Event code: 4005
Event message: Forms authentication failed for the request. Reason: The ticket supplied has expired.
Event time: 8/11/2009 11:02:51 AM
Event time (UTC): 8/11/2009 3:02:51 PM
Event ID: ff1e3b115de44ef28004be512c72db6f
Event sequence: 421772
Event occurrence: 82
Event detail code: 50202
Application information:
Application domain: /LM/W3SVC/1/ROOT/NWOChat-1-128943816205520000
Trust level: Full
Application Virtual Path: /NWOChat
Application Path: **********
Machine name: ********
Process information:
Process ID: 4020
Process name: w3wp.exe
Account name: ***************************