c# throw webexception and socketexception after running for a while

Question

I wrote a C# program which post HTTP heartbeat request every 5 second,it runs well at the beginning,but after about 20-40 mins, it keeps throwing webException at the method HttpWebRequest.BeginGetRequestStream( ) ( or at HttpWebResponse.EndGetResponse() if I don't need to post data).

I met this problem in some specific computers, whose performance wasn't that good.

I've made several attempts,but still get the same result:

1. change another server address
2. use socket to post http request, and get socketException when new a socket(before socket.connect())
3. asynchronous and synchronous http request
4. use .net 4.0 (I use .net3.5 currently)
5. almost the same code in winForm and WPF program
6. make sure do nothing with proxy or firewall

here is the exception stacktrace:

   System.Net.WebException: the underlying connection was closed:unable to connect to the remote server. 
---> System.Net.Sockets.SocketException: an invalid argument was supplied.
at System.Net.Sockets.Socket..ctor(AddressFamily addressFamily, SocketType socketType, ProtocolType protocolType)
at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6, Int32 timeout)
at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32 timeout, GeneralAsyncDelegate asyncCallback)
at System.Net.Connection.CompleteStartConnection(Boolean async, HttpWebRequest httpWebRequest)

here is the kernal part of my code:

    public MainWindow()
    {
        InitializeComponent();
        timer = new Timer(new TimerCallback(delegate (object obj)
        {
            try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                request.Method = "GET";
                request.KeepAlive = false;
                IAsyncResult result = request.BeginGetResponse(new AsyncCallback(ResponseCallback), request);
                ThreadPool.RegisterWaitForSingleObject(result.AsyncWaitHandle, new WaitOrTimerCallback(TimeoutCallback), request, 15000, true);
            }
            catch(Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }), null, Timeout.Infinite, Timeout.Infinite);
    }

    private void TimeoutCallback(object state, bool timeout)
    {
        if (timeout)
        {
            HttpWebRequest request = state as HttpWebRequest;
            if (request != null)
            {
                request.Abort();
            }
        }
    }

    private void ResponseCallback(IAsyncResult result)
    {
        try
        {
            HttpWebRequest request = (HttpWebRequest)result.AsyncState;
            HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(result);

            Stream responseStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(responseStream);
            string responseString = reader.ReadToEnd();
            responseStream.Close();
            reader.Close();
            response.Close();

            Console.WriteLine(responseString);
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

I don't think there is anything wrong with the code, maybe is the .net problem or network problem, I am very grateful if someone give me some advices.