SerialPort timeouts in ReadChar() method, WPF, "The operation has timed out."

Question

Goal of my program is to send data to the RS232 and read them in with a different function. I have a direct connection (wire) between the RS232 pin 2 and pin 3. I'm getting randomly occurring timeouts in WPF when I'm reading the serial port, but I receive the data and can print it.

Create a serial port and a delegate in my class.

        SerialPort _serialPort;
        `public delegate void ReceiveData(object sender);`

Create a SerialDataReceivedEventHandler after InitializeComponent() _serialPort.DataReceived += new SerialDataReceivedEventHandler(SerialPort1_DataReceive);

Define method for event.

        private void SerialPort1_DataReceive(object sender, SerialDataReceivedEventArgs e)
        {

            Dispatcher.BeginInvoke(new ReceiveData(OutputDataToList), sender);
        }

Define method for the delegate.

        private void OutputDataToList(object sender)
        {
            SerialPort sp = (SerialPort)sender;
            _inChar = '1';
            while (_inChar != '\n')
            {
                try
                {
                    _inChar = (char)sp.ReadChar();
                }
                catch (Exception ex)
                {
                    MessageBox.Show("Error while receiving the serial port. " + ex.Message);
                    break; 
                }
                if (_inChar != '\n')
                {
                    _inMessage += _inChar;
                }
            }
            if (_inMessage != "")
            {
                lstReads.Items.Add(_inMessage);
                lstReads.SelectedIndex = lstReads.Items.Count - 1;
            }
            _inMessage = "";
        }

In 70%-90% of the tests, the program works fine but sometimes, this error occurs:

I changed the timeout time. It gets better (but still happens sometimes) for shorter time but I don't understand why.

            _serialPort.ReadTimeout = 500;
            _serialPort.WriteTimeout = 500;