谁有遇到过httpwebrequest模拟登录出现(503) Server Unavailable.这个错误？

httpwebrequest模拟登录出现The remote server returned an error: (503) Server Unavailable.这个错误以前可以通过软件模拟登录，今天突然发现不行了，出现这个错误，尝试adsl断网重连了，还是不行，但是用浏览器就可以正常登录，电脑用了代理后，软件又可以正常登录了，下面是我请求的部分代码                request = (HttpWebRequest)HttpWebRequest.Create(url);
                request.Headers[HttpRequestHeader.Cookie] = strCookie;
                request.CookieContainer = cookieContainer;
                request.Proxy = WebProxy.GetDefaultProxy(); ;
                request.ProtocolVersion = HttpVersion.Version10;
                request.ContentType = contentType;
                request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
                if (referer != "")
                    request.Referer = referer;
                request.UserAgent ="Mozilla/5.0 (Windows NT 5.1; rv:5.0.1) Gecko/20100101 Firefox/5.0.1";
                request.Method = "POST";
                request.ContentLength = byteRequest.Length;
                using (Stream stream = request.GetRequestStream())
                {
                    stream.Write(byteRequest, 0, byteRequest.Length);
                }
                response = (HttpWebResponse)request.GetResponse();