auth loop in asp.net 2

 I've got a web application built in C# running under .net 2.  It's a secure website using Forms Authentication with the userId and password sent to us in the query string.  I have an auth.aspx page that handles the authentication and then uses the  FormsAuthentication.RedirectFromLoginPage(id, false);
method.  
 In most installations this works fine, but in a few sites and in those only occasionally we'll see this in the iis logs repeatedly, like 20-30 times.  One site where it isn't working occasionally is on server 2003 and it is a server in a domain, but not part of a cluster I'm told.

 
2009-02-25 13:16:12 W3SVC1685255489 1.99.99.37 GET /Controller.aspx requestType=htmlComponent&loginId=cbrannan 82 - 172.20.10.127 Mozilla/4.0+ 302 0 0
2009-02-25 13:16:12 W3SVC1685255489 1.99.99.37 GET /Auth.aspx ReturnUrl=%2fController.aspx%3frequestType%3dhtmlComponent%26loginId%3dcbrannan 82 - ...