IE HTTP close (reset) - port reuse causing firewall issues

Question

Having an issue with some systems reusing the same TCP port number between sessions, causing the firewall to drop the connection.
Internet Explorer is creating the HTTP socket connection to port 80. An ephemeral port (assigned by Windows) is bound to the local side of the connection. The first connection goes through just fine. The socket is
closed/reset. However, the very next connection (hundreds of milliseconds later), is using the same ephemeral port, causing the firewall to discard the connection.
I have tried setting TcpTimedWaitDelay in the registry but that did not help. Since the socket is being reset, it never goes into the TIME_WAIT state.
Any suggestions? This does not happen consistently - on the order of 10s of times per day.
Thanks!

Answer 1

Problem is still occurring. Customer has built a new client system with MS-only software (no virus protection, etc.). Upgraded this system to IE9.  Problem is still occurring. Tried disabling NativeXMLHTTP option but no difference.
Here is the ASP VBScript code that causes the error to appear:
function SubmitPost(data,ErrHow)
var d = new Date();
return SendData('POST','TDMaster.asp?InstID=' + document.getElementById("tdInstance").value + '&UID=' + d.getTime(),data,ErrHow,0);
//Returns valid version of MSXML
function GetMSXML()
var progIDs = ['Msxml2.XMLHTTP.6.0','Microsoft.XMLHTTP'];
        for (var i = 0; i < progIDs.length; i++) {
            try {
                var http = new ActiveXObject(progIDs[i]);
                return http;
            catch (ex) {
        return null;
// Function that actually sends the data and returns the response
// Format 0 = XML
// Format 1 = Binary
var http;
var timedOut;
function SendData(method,url,data,ErrHow,Format)
            http =  GetMSXML() ; 
            var ResultXML;
            var e;
            http.open(method, url, false);
            http.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
            http.setRequestHeader("Content-Length", data.length);
            try {
                        http.send(data);
                        if(Format == 0) {
                            return http.responseText;
                        } else {
                            return http.responseBody;
            } catch(e) {
                                return CreateError(e.number, e, ErrHow);