C# - GetRequestStream Throws Timeout Exception Randomly

After googling for couple of days, I really cannot solve described issue. Hope here will find a solution

I'm using attached code when calling WCF service on the same server. I get Timeout error randomly in call WebReq.GetRequestStream()

When I'm check netstat I see that connection remains open, so probably is there a problem, but I don't know how to solve it

//request inicialization
HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(url);
WebReq.Method = "POST";
WebReq.ContentType = "application/json; charset=utf-8";
WebReq.ContentLength = buffer.Length;
WebReq.Proxy = null;
WebReq.KeepAlive = false; //also tried with true
WebReq.AllowWriteStreamBuffering = false; //also tried with true
//this produces an error
using (Stream PostData = WebReq.GetRequestStream())
{
PostData.Write(buffer, 0, buffer.Length);
PostData.Close();
}
//open and read response
HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();
Stream Answer = WebResp.GetResponseStream();
StreamReader _Answer = new StreamReader(Answer);
WebResp.Close();
//return string
return _Answer.ReadToEnd();

Timeout is thrown mostly after some 10 seconds of idle time, but also after five or so requests in the row. Really cannot find a pattern.

What could be wrong with this code? Is there any other (better) way for calling WCF service?