Junk characters coming in from pinvoke WM_GETTEXT

I have a method set that uses pinvoke to call WM_GETTEXT on another program's textbox - and it works fairly well, but frequently I just get back total junk text appended to the end of it. (The ORIGINAL text is always intact.)

This is random, I cannot reproduce it on demand, but it is frequent enough to be stopship.

Here is the text to get the information.

System.Text.StringBuilder strBuffer = new System.Text.StringBuilder();

int nLen = 0;

bool nUpdated = false;

try
{
    this.isOpen = false;

    if (ptrHandle == null)
        return;

    if (ptrHandle == IntPtr.Zero)
        return;

    nLen =
        Converter.SendMessage(ptrHandle, Converter.WM_GETTEXTLENGTH, 0, 0);

    if (nLen <= 0)
        return;

    if (nPreviousLen != nLen)
        nUpdated = true;

    if (nUpdated)
    {
        System.Diagnostics.Debug.WriteLine("nLen:\t{0}", nLen);

        strBuffer = new System.Text.StringBuilder(null, nLen + 1);

        System.Diagnostics.Debug.WriteLine("strBuffer:\t{0}", strBuffer.ToString());

        int sLen = Converter.SendMessageByString(ptrHandle, Converter.WM_GETTEXT, nLen
            , strBuffer);

        System.Diagnostics.Debug.WriteLine("sLen:\t{0}", sLen);

        System.Diagnostics.Debug.WriteLine("\n\nstrBuffern\n\n{0}", strBuffer.ToString());

        strBuffer = new System.Text.StringBuilder(strBuffer.ToString().Left(sLen));

        System.Diagnostics.Debug.WriteLine("\n\nsLenBuffer\n\n{0}", strBuffer.ToString());

source = new Special.IO.TextReader( 
                    new System.IO.MemoryStream(  System.Text.Encoding.Default.GetBytes(strBuffer.ToString() ) ), nUpdated );
        }
    }
}


    /// <summary>
    /// Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
    /// <br />
    /// To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread's message queue and return immediately, use the PostMessage or PostThreadMessage function.
    /// </summary>
    /// <param name="hWnd">
    /// Handle to the window whose window procedure will receive the message. 
    /// If this parameter is HWND_BROADCAST, the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
    /// </param>
    /// <param name="Msg">
    /// [in] Specifies the message to be sent.
    /// </param>
    /// <param name="wParam">
    /// [in] Specifies additional message-specific information.
    /// </param>
    /// <param name="lParam">
    /// [in] Specifies additional message-specific information.
    /// </param>
    /// <returns>
    /// The return value specifies the result of the message processing; it depends on the message sent.
    /// </returns>
    [DllImport("user32.dll", EntryPoint = "SendMessageA", CharSet = CharSet.Ansi, SetLastError = false)]
    internal static extern int SendMessageByString(IntPtr hWnd, uint Msg, int wParam, StringBuilder lParam);

    /// <summary>
    /// Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
    /// <br />
    /// To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread's message queue and return immediately, use the PostMessage or PostThreadMessage function.
    /// </summary>
    /// <param name="hWnd">
    /// Handle to the window whose window procedure will receive the message. 
    /// If this parameter is HWND_BROADCAST, the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
    /// </param>
    /// <param name="Msg">
    /// [in] Specifies the message to be sent.
    /// </param>
    /// <param name="wParam">
    /// [in] Specifies additional message-specific information.
    /// </param>
    /// <param name="lParam">
    /// [in] Specifies additional message-specific information.
    /// </param>
    /// <returns>
    /// The return value specifies the result of the message processing; it depends on the message sent.
    /// </returns>
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    internal static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);