Security :: Accessing Reporting Application From External Network?

We have an issue with an internal application, where accessing it from LAN (private n/w) works fine, while access from external n/w throws up erratic behavior.

The Current System :

an aspx page (asp version 1.0) hosted on the IIS server (6.1). The front end is a vb form that connects to a sql 2000 database.

Users' credentials authorized using NTLM security.

Access from an external n/w - a reverse proxy configured with an ISA server 2006 to authenticate credentials.

Domain Controller - Windows Server 2008 R2 x64

The issue when accessed from external n/w :

everytime an entry from a drop-down is selected from an Ajax enabled control, instead of the fields populating, the page gets refreshed and the content of the box are cleared.

Also, clicking on other items that normally hyperlink to other pages, results in nothing happening.

Upon investigation, we realized that :

Everytime it fails, a string - "&AuthResend" followed by an alpha numeric string is attached, which attempts to authenticate the user's credentials everytime a request is made to the server.

ISA error logs yielded the following error msg :

"12210 Internet Server API ( ISAPI ) has finished handling the request. Contact your system administrator. "

This behavior is inconsistent except when accessed from Google Chrome, where it fails each time.

We have applied all the recommended hot fixes from Microsoft's KB pages.