How to control flickering problem in C# windows application to display data from serial port?

private void button1_Click(object sender, EventArgs e)
        {
            SerialPort mySerialPort = new SerialPort("COM1");

            mySerialPort.BaudRate = 9600;
            mySerialPort.Parity = Parity.None;
            mySerialPort.StopBits = StopBits.One;
            mySerialPort.DataBits = 8;
            mySerialPort.Handshake = Handshake.None;
            mySerialPort.RtsEnable = true;

            SerialPort sp = (SerialPort)sender;
            label1.Text = sp.ReadExisting();
            mySerialPort.Open();
            mySerialPort.Close();
        }

public partial class MainForm : Form
    {
        SerialPortManager _spManager;


        
        public MainForm()
        {
            InitializeComponent();
            
            DoubleBuffered = true;
            UserInitialization();
                       
        }


        
        private void UserInitialization()
        {
            _spManager = new SerialPortManager();
            SerialSettings mySerialSettings = _spManager.CurrentSerialSettings;
            serialSettingsBindingSource.DataSource = mySerialSettings;
            portNameComboBox.DataSource = mySerialSettings.PortNameCollection;
            baudRateComboBox.DataSource = mySerialSettings.BaudRateCollection;
            dataBitsComboBox.DataSource = mySerialSettings.DataBitsCollection;
            parityComboBox.DataSource = Enum.GetValues(typeof(System.IO.Ports.Parity));
            stopBitsComboBox.DataSource = Enum.GetValues(typeof(System.IO.Ports.StopBits));

            _spManager.NewSerialDataRecieved += new EventHandler<SerialDataEventArgs>(_spManager_NewSerialDataRecieved);
            this.FormClosing += new FormClosingEventHandler(MainForm_FormClosing);
        }

        
        private void MainForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            _spManager.Dispose();   
        }

        void _spManager_NewSerialDataRecieved(object sender, SerialDataEventArgs e)
        {
            if (this.InvokeRequired)
            {
                
                this.BeginInvoke(new EventHandler<SerialDataEventArgs>(_spManager_NewSerialDataRecieved), new object[] { sender, e });
                return;
            }

            int maxTextLength = 1000; // maximum text length in text box
            if (tbData.TextLength > maxTextLength)
                tbData.Text = tbData.Text.Remove(0, tbData.TextLength - maxTextLength);

           
            string str = Encoding.ASCII.GetString(e.Data);
            tbData.AppendText(str);
            tbData.ScrollToCaret();
            DoubleBuffered = true;
            label1.Text = str;
            


        }

        
        private void btnStart_Click(object sender, EventArgs e)
        {
            _spManager.StartListening();
        }

        
        private void btnStop_Click(object sender, EventArgs e)
        {
            _spManager.StopListening();
        }
    }