problems renewing logins

Has anyone had problems with sustaining a nocat login?

We're using firefox (mostly) and we have popups enabled/allowed from our nocat 
gateway.

After our browser is "captured" we login successfully in the pop-up window but 
find that *sometimes* our client computer seems to get "cut off" by the 
gateway and our browser window gets recaptured; leaving both the original 
pop-up window that still says we are logged in *plus* the nocat login window 
appears in another broswer window, perhaps the one we were last using.

It is as if the pop-up window is not doing its job properly.

The odd thing is that some users do not have this problem, yet others often do.

Any ideas please?

Steve Platt