User.Identity.Name sometimes blank with Windows authentication

[AOL]

My web app uses Windows authentication. All other authentication methods are disabled.It works at first; User.Identity.Name returns "MYDOMAIN\myuser." However, if I wait 131 seconds (yes, I timed it, though I'm not sure it always takes exactly that long) and reload the page, User.Identity.IsAuthenticated = false and User.Identity.Name = "". At this point I can just reload the page and it works again, for another 131 seconds of inactivity.Relevant parts of web.config:\[code\]<authentication mode="Windows" /><authorization> <deny users="?" /> </authorization><identity impersonate="false" />...<security> <authentication> <windowsAuthentication enabled="true" /> <anonymousAuthentication enabled="false" /> </authentication></security>\[/code\]More information, possibly relevant:My solution is deployed to two different IIS applications. The first one is for internal users; it just uses Windows authentication. The second one is for external users; it uses anonymous authentication (in the root folder only) and forms authentication. I'm only having problems with the internal site.