WebBrowser(in .Net 2.0 WinForm) hangs when waiting for asynchronous pluggable protocol response

Question

I write a temp http protocol according to http://www.codeproject.com/KB/aspnet/AspxProtocol.aspx?display=PrintAll and the Code is as follow,

public void Start(string szURL, IInternetProtocolSink Sink, IInternetBindInfo pOIBindInfo, uint grfPI, uint dwReserved) 
        { 
            try 
            { 
                IServiceProvider Provider = (IServiceProvider)Sink; 
                object obj_Negotiate = new object(); 
                Provider.QueryService(ref Guids.IID_IHttpNegotiate, ref Guids.IID_IHttpNegotiate, out obj_Negotiate); 
                IHttpNegotiate Negotiate = (IHttpNegotiate)obj_Negotiate; 
                string strRequestHeaders; 
                Negotiate.BeginningTransaction(szURL, string.Empty, 0, out strRequestHeaders); 
                BINDINFO BindInfo = GetBindInfo(pOIBindInfo); 
                String RawPostData = GetPostData(BindInfo, szURL); 
                HttpWebResponse HttpResp = GetHttpWebResponse(szURL, RawPostData, strRequestHeaders); 
                uint RespCode = (uint)HttpResp.StatusCode; 
                using (Stream RespStream = HttpResp.GetResponseStream()) 
                { 
                    byte[] rgb = new byte[1024]; 
                    int cb; 
                    while ((cb = RespStream.Read(rgb, 0, rgb.Length)) > 0) 
                    { 
                        MemStream.Write(rgb, 0, cb); 
                    } 
                } 
                MemStream.Position = 0; 
                String ContentType = HttpResp.GetResponseHeader("Content-Type"); 
                string StrResponseHeaders = HttpResp.Headers.ToString(); 
                string strNewResponseHeaders; 
                Negotiate.OnResponse(RespCode, StrResponseHeaders, strRequestHeaders, out strNewResponseHeaders); 
                Sink.ReportData(BSCF.BSCF_FIRSTDATANOTIFICATION | BSCF.BSCF_LASTDATANOTIFICATION | BSCF.BSCF_DATAFULLYAVAILABLE, (uint)MemStream.Length, (uint)MemStream.Length); 

                if (!String.IsNullOrEmpty(ContentType)) 
                { 
                    Sink.ReportProgress((uint)BINDSTATUS.BINDSTATUS_MIMETYPEAVAILABLE, ContentType); 
                } 
                Sink.ReportResult(0, 0, null); 
                HttpResp.Close(); 
            } 
            catch (Exception e) 
            { 
                Debug.WriteLine(e.ToString()); 
            } 
        } 

it works well for web pages navigating, but the WebBrowser and the winform hangs when waiting for a slow asynchronous pluggable protocol response . such as the target file sleep.php :

<?php
sleep(10);
print_r("Sleep Test\n");
?>

The winform will not reponse any mouse click within 10 seconds. How to resolve it (let the winform response the mouse click when pluggable protocol is in progress) ? Thank you.

Envrionment: .Net 2.0, VS2008, C#, WinForm, WebBrowser Control