Is this happening on an XP Pro development machine? Sometimes, if you have many javascripts/images/css files on a page as well as IFrames and multiple AJAX calls, your one page will run out of connections in a development environment. There are two ways to overcome this.
Switch off HTTP Keep-Alives. In IIS manager, go to the Web Site tab and under connections, uncheck the HTTP Keep-Alives Enabled checkbox. Don't do this in a production environment as it will slow down your site a lot.
Increase the number of concurrent connections.
You’re limited by default to 10 concurrent connections by design, but this can be increased. First, make sure your default windows script host is set to the console (cscript.exe) one. In a command prompt run:
cscript //h:cscript
Next, let’s increase the connection limit to 40.
C:\Inetpub\AdminScripts\adsutil set w3svc/MaxConnections 40
You can make the IIS connection timeout more aggressive so connections don’t last as long.
Set the default script host back to windows:
cscript //h:wscript