Apache times out after 2 minutes, when Apache TimeOut is 1200

We have a set up where the browser makes an http request to Box A which in turn makes an http request to Box B. What we're encountering is that Box A waits for Box B to respond for two minutes then the users sees:

The page cannot be displayed
Explanation: There is a problem with the page you are trying to reach and it cannot be displayed.

Try the following:

    * Refresh page: Search for the page again by clicking the Refresh button. The timeout may have occurred due to Internet congestion.
    * Check spelling: Check that you typed the Web page address correctly. The address may have been mistyped.
    * Access from a link: If there is a link to the page you are looking for, try accessing the page from that link. 

Technical Information (for support personnel)

    * Error Code: 404 Not Found. The requested item could not be located. (12028)

Watching the logs on Box B we see that it takes 5 minutes to do the work requested.

The problem is that the apache time outs on both boxes are set 1200 (20m), not 120 (2m).

Any ideas where to look?

Edit: Box A handles the request by running some Perl code which makes an HTTP request to Box B.

From serverfault Robert Gowland

• If this is from a browser. I suspect it's a browser-side timeout. In some browsers it's configurable, see for example http://support.microsoft.com/kb/813827 for how to set it for Internet Explorer.

  One workaround could be to periodically send some data, such as an HTML comment, to the browser in order to keep the connection alive.

  Robert Gowland : @Kamil, That's a really good point, and one that we hadn't considered. However, testing shows that's not the issue. Still, +1 for helping us rule out the browser.
  From Kamil Kisiel

• I was able to determine that the response I was encountering was generated by Microsoft's ISA firewall, nothing to do with Apache or even our boxes at all. I didn't realize our customer had our boxes behind a firewall or even that firewalls had timeout settings.

  From Robert Gowland