I'm having some issues with what I think may be caused by serial packet collision or packets getting clipped. 

I have an ATMEL 168 based microcontroller that I am communicating with through the SerialPort object in a C# Windows Form Application. The board has 5 IO pins, 3 Analog and 2 PWM. I have two TrackBar Controls on the form to control the PWM value (0-255), one checkbox to toggle the polling for the inputs and another TrackBar to control the Interval (30 - 2000 ms) of the Timer. What happens is that as soon as I enable the Timer to start the polling, the servos (PWM) start "twitching" as I scroll the TrackBars. 

I trimmed down the code below for clarity's sake but I've tried checking if BytesToRead is equal to 0 prior to issuing another SerialPort.Read, in addition to doing the same for SerialPort.Write.

  public partial class Form2 : Form
  {
    System.IO.Ports.SerialPort port = new System.IO.Ports.SerialPort();
    System.Timers.Timer tmrPolling = new System.Timers.Timer(200);
    static byte[] inputPacket = new byte[2] { (byte)255, (byte)0 }; // Byte to send to SerialPort when input polling timer is activated.

    public Form2()
    {
      InitializeComponent();
      port.PortName = "COM9";
      port.BaudRate = 38400;
      port.DataBits = 8;
      port.StopBits = System.IO.Ports.StopBits.One;
      port.Parity = System.IO.Ports.Parity.None;
      port.ReadTimeout = 1;
      port.ReceivedBytesThreshold = 5;
      port.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(port_DataReceived);
      port.Open();      
      tmrPolling.Elapsed += new System.Timers.ElapsedEventHandler(tmrPolling_Elapsed);     
    }

    void tmrPolling_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
    {  
      port.Write(inputPacket, 0, 2); // writes the input packet to the SerialPort - needed to read the IO values
    }
    
    void port_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
    {
      // Reads the data coming in from the serial port and calls the thread safe delegate to update the values on the form.
      byte[] received = new byte[port.BytesToRead];
      port.Read(received, 0, received.Length);          
    }

    private void UpdateValues()
    {
      byte[] b = new byte[] { (byte)255, (byte)6, (byte)hsbPan.Value, (byte)vsbTilt.Value, (byte)0,       
        (byte)0, (byte)0, (byte)0, (byte)13, (byte)10 };
      port.Write(b, 0, 9);
    }

    private void chkEnablePolling_CheckedChanged(object sender, EventArgs e)
    {     
      tmrPolling.Enabled = chkEnablePolling.Checked;     
      vsbInputInterval.Enabled = chkEnablePolling.Checked;
      vsbInputInterval.Value = Convert.ToInt32(tmrPolling.Interval);
      txtInputInterval.Enabled = chkEnablePolling.Checked;
      txtInputInterval.Text = ((float)tmrPolling.Interval / (float)1000).ToString() + " sec";
    }

    private void vsbInputInterval_Scroll(object sender, ScrollEventArgs e)
    {
      // adjusts the tick frequency of the Timer for input polling. 
      txtInputInterval.Text = ((float)tmrPolling.Interval / (float)1000).ToString() + " sec";
      tmrPolling.Interval = vsbInputInterval.Value;
    }    

    private void vsbTilt_Scroll(object sender, ScrollEventArgs e)
    {
      UpdateValues();
    }

    private void hsbPan_Scroll(object sender, ScrollEventArgs e)
    {
      UpdateValues();
    }
  }