WebRequest.GetResponse intermittent timeouts

Question

I am using some inherited code to access a trading API and get intermittent timeouts at the .GetResponse call.

As its intermittent I did think its resource related - other articles refer to the importance of closing disposable objects, so I have used a Using block around the .GetResponse. Have I dealt with all disposables ?

I'm quite new to Web/Http calls so any help please !

    Dim success As Boolean = False

    Do Until success = True
        Try
            Dim request = WebRequest.Create("https://poloniex.com/tradingApi")
            Dim nonce As Long = CLng((DateTime.Now - New DateTime(1970, 1, 1)).TotalMilliseconds) + nonceoffset
            Dim postData = [String].Format("command={0}&nonce={1}", method, nonce)
            postData = postData & params

            Dim hmAcSha = New HMACSHA512(Encoding.ASCII.GetBytes(PrivateKey))
            Dim messagebyte = Encoding.ASCII.GetBytes(postData)
            Dim hashmessage = hmAcSha.ComputeHash(messagebyte)
            Dim sign = BitConverter.ToString(hashmessage)
            sign = sign.Replace("-", "")

            request.Timeout = 5000
            request.ContentType = "application/x-www-form-urlencoded"
            request.ContentLength = messagebyte.Length
            request.Method = "POST"
            request.Headers.Add("Key", PublicKey)
            request.Headers.Add("Sign", sign.ToLower())

            System.Net.ServicePointManager.Expect100Continue = False

            Dim stream As System.IO.Stream
            stream = request.GetRequestStream()
            stream.Write(messagebyte, 0, messagebyte.Length)
            Dim json As String
            Using response As WebResponse = request.GetResponse()
                Dim postreqreader = New StreamReader(response.GetResponseStream())
                json = postreqreader.ReadToEnd()
            End Using

            success = True
            Return json

        Catch e As WebException
            DebugTextAdd("ALERT! API WebException : " & e.Message, MethodBase.GetCurrentMethod.Name())
            If System.Threading.Interlocked.Decrement(maxRetryCount) = 0 Then
                DebugTextAdd("ALERT! API call failed after several tries. Return Nothing", MethodBase.GetCurrentMethod.Name())
                success = True
                Return Nothing
            Else
                DebugTextAdd("Attempt #" & maxRetryCount.ToString & ", try again", MethodBase.GetCurrentMethod.Name())
            End If

        Catch e As Exception
            DebugTextAdd("ALERT! API exception : " & e.Message, MethodBase.GetCurrentMethod.Name())
            If System.Threading.Interlocked.Decrement(maxRetryCount) = 0 Then
                DebugTextAdd("ALERT! API call failed after several tries. Return Nothing", MethodBase.GetCurrentMethod.Name())
                success = True
                Return Nothing
            Else
                DebugTextAdd("Attempt #" & maxRetryCount.ToString & ", try again", MethodBase.GetCurrentMethod.Name())
            End If

        Finally

        End Try
    Loop