System.Web.HttpContext.Current.User.Identity.IsAuthenticated fails sometimes

I have been having trouble with my production site (not my development sites). Every now and then both Firefox and Chrome fail to log users in (all users both on our client network and general web). But the strange part is that Internet Explorer always works correctly and has NEVER failed once (I have delete cache and cookies in browsers but still the same thing happens).

How to restrict unlogged/unauthorized users from viewing web pages in ASP.NET

I have some created web forms and I need to check whether the user is authenticated or not, before displaying the other web forms. All the users can access Default.aspx and About.aspx pages.
And I have three types of users namely- Admin,User and Super User. Also, I keep the authentication details in my own SQL server db.