All users use the same page, and go thru the same script, and I am not using any cookies for this.
The particulars of the page (intranet) includes using ServerVariables to find User logon ID (NT authentification), then execute a stored procedure and a SQL query using this ID.
I am checking to see if ServerVariables is the cause, since I remembered somewhere someone has mentioned the lack of 100% reliability of it. Also, I am checking anything related to database access, and I am also increasing the ScriptTimeOut to 300 from 90.
I'll see if this works.