Asynchronous client script callbacks for iFrames

I'm have an existing ASP.NET application.
In this application i have a web page that contains an iFrame.
This iFrame displays a url link (one of many) that is stored in a text file.  i.e. the text file contains a list of many url's
for example
http://screen1.aspx
http://screen2.aspx
http://screen3.aspx
http://screen4.aspx

and on page load using StreamReader class of the .NET framework I loop through each line item of the text file. read the url and store it in a session variable. and pass the first value in the session which in this case is http://screen1.aspx to the iFrame as url to load it for the first time. Next I'm making use of a javascript postback function to reload the page and this time i'm passing the next url to the iFrame so that it now displays this url. i.e http://screen2.aspx

This works fine so far.. however i have some challenges to overcome.. namely..

1. The url to load i.e. for example:- http://screen2.aspx may takes a long time to load..
2. the url mentioned in the text file i.e. for example:- http://screen3.aspx  may not exist.
3. there may be an error in the page while loading the page. i.e. for example:- http://screen4.aspx 

So i have to come up with a different approach to do this.. this is where it gets tricky..

I need to use client callback script (i think) to do a background processing that will asynchronous load the next url i.e. http://screen2.aspx to load in a second iFrame in the background and then display it when the time comes to do so i.e. on javascript postback function without encountering problem 2 and 3 mentioned above. and if it encounters problem 2 and 3 then the background iFrame must asynchronously load the next url in line from the text file. i.e. http://screen3.aspx and so on and so forth.. So that the page displays new screens in the iFrame almost asynchronously without a need of a postback

How can i do this ???

Please help me.. any help in this direction will be greatly appreciated..