Httpwebrequest error

Question

I’m using this code below to send a xml to the webservice from city hall, the maximum RPS you send is 40. What happens is that I already sent with 40 and did not get problems earlier. It is generated only once a month, when I Gero for example, I am trying to generate with 13 RPS, it returns me a generic error:

An error occurred while sending the request. ; The server returned an invalid or unrecognized Response.

So I can not know where the problem is, I thought it could be in the code, but with 1 RPS per batch sent correctly, and with 5 also.

 private string postB(string url, string method, string xml, string numero, int idempresa, string ws)
    {
        try
        {
            XmlDocument doc = new XmlDocument();

            doc.Load(xml);

            HttpWebRequest webRequest = getWebRequestB(url, method);
            webRequest.Timeout = Timeout.Infinite;
            //webRequest.KeepAlive = true;
            webRequest.Proxy = HttpWebRequest.GetSystemWebProxy();
            webRequest.UseDefaultCredentials = true;
            webRequest.Proxy.Credentials = CredentialCache.DefaultCredentials;
            WebProxy myProxy = new WebProxy();
            Uri myUri = new Uri("http://192.168.0.3:3128");
            myProxy.Address = myUri;
            myProxy.Credentials = CredentialCache.DefaultCredentials;
            webRequest.Proxy = myProxy;


            montaEnvelopeB(webRequest, doc, numero, xml, idempresa, ws);

            return getSoapResponseB(webRequest);
        }
        catch (WebException webex)
        {
            WebResponse errResp = webex.Response;
            if (errResp != null)
            {
                using (Stream respStream = errResp.GetResponseStream())
                {
                    StreamReader reader = new StreamReader(respStream);

                    msgm = reader.ReadToEnd();
                    cod = "Erro";
                }
            }
            else { cod = "Erro";  msgm = webex.ToString(); }

            return msgm;
        }
    }

I’ve tried to put with webRequest.KeepAlive = true; but did not give, nor putting with false.