Popup window and authentication: URGENT Please!!

[VS_NET_DEV]

Hi all,
I am running asp.net on IIS 6.0 /IE 6.0 on Windows 2003 server.
I have form authenication and in Global.asax I have:
private void Global_AcquireRequestState(object sender,
System.EventArgs e)
{

if (Session["user_id"] == null)
{
if (!Request.Path.ToUpper().EndsWith("LOGIN.ASPX"))
{
string url =
string.Format(
// we build the whole link. Firstly, we get our host name
"http://" + Request.Headers["Host"] +
// then the path of the request
Request.ApplicationPath +
"/MainPage/Login.aspx"
);
Response.Redirect( url);
//omitted

When authenticated the session will have user_id value. This works
great on regular windows. But in Javascript popup windows fails -
about 10% of the the time I am redirected to the login page
again!!!!!!
Why?? it happens randomly about 10% of the time. If u try again, it
is working.
Thanks in advance!
Jenny -