C# 的POST出现以下的异常

public string Post(string url, string content, string referer, Encoding endcode)
        {
            int failedTimes = _tryTimes;
            //endcode = Encoding.UTF8;
            while (failedTimes-- > 0)
            {
                try
                {
                    if (_delayTime > 0)
                    {
                        Thread.Sleep(_delayTime * 1000);
                    }
                    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(new Uri(url));
                    req.CookieContainer = _cc;
                    req.Referer = referer;
                    byte[] buff = Encoding.GetEncoding("GB2312").GetBytes(content);
                    req.Method = "POST";
                    req.Timeout = _timeout;
                    req.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; CIBA)";
                    //req.CookieContainer = "";                    req.ContentType = "application/x-www-form-urlencoded";
                    req.ContentLength = buff.Length;
                    if (null != _proxy && null != _proxy.Credentials)
                    {
                        req.UseDefaultCredentials = true;
                    }
                    req.Proxy = _proxy;
                    //req.Connection = "Keep-Alive";                    Stream reqStream = req.GetRequestStream();
                    reqStream.Write(buff, 0, buff.Length);
                    reqStream.Close();
                    //接收返回字串
                    HttpWebResponse res = (HttpWebResponse)req.GetResponse();
                    //StreamReader sr = new StreamReader(res.GetResponseStream(), Encoding.UTF8);
                    StreamReader sr = new StreamReader(res.GetResponseStream(), endcode);
                    return sr.ReadToEnd();
                }
                catch (Exception e)
                {
                    TraceLog.Error("HTTP POST Error: " + e.Message);
                    TraceLog.Error("Url: " + url);
                    TraceLog.Error("Data: " + content);
                }
            }            return string.Empty;
        }基本上能POST成功，但是有的时候POST后出现一个这样的异常消息，寻了很久的资料，没找到相应的解决方法：HTTP POST Error: The underlying connection was closed: An unexpected error occurred on a receive.