Is it possible that the particular client has different proxy settings than
other clients?  If so, the proxy server may be dropping the connection after
the time expires.  I haven't run into this exactly, but my guess is that you
can reset the proxy connection by creating a new WebProxy object and use the
following code:

WebProxy proxyObject = new WebProxy("http://proxyserver:80/",true);
GlobalProxySelection.Select = proxyObject;

This should set the new proxy object to be used for all HTTPRequest calls.

I do not know for certain if this will work, so post back if it does... it
would be interesting to know.

Hope this helps,
--- Nick