Retrieving the current authorized user

I am having an issue on two different servers with retrieving the current authorized user on two different users in staging and dev. the issue seems intermittant and I have tried two different methods for retrieving the authorized user both with the same results. Any ideas on what might be causing this or what I might not have configured correctly?



1) MyUser= Request.ServerVariables["AUTH_USER"];


 2)MyUser =  HttpContext.Current.User.Identity.Name.ToString();
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