asp.net web application fails to recognize user identity

We have a web application and it uses System.Web.UI.Page.User.Identity to determine who the current user is. We are experiencing an intermittent problem, where a user is not recognized (their identity is blank or empty). Anonymous authentication is disabled so I don't see how they can fully access the web application without it recognizing who they are. This happens like once a month for half a day, and then it fixes itself. So it's really hard to diagno开发者_JAVA百科se but I would really like to track down the issue. Any suggestions? Thank you!

Make sure you have anonymous authentication turned off & windows authentication enabled for the application in IIS.