RS232 communication and application freeze

Private Sub initCommPort()
    If SerialPort1.IsOpen = False Then
        SerialPort1.PortName = ListBox1.SelectedItem
        SerialPort1.BaudRate = 115200
        SerialPort1.Parity = Parity.None
        SerialPort1.DataBits = 8
        SerialPort1.StopBits = StopBits.One
        SerialPort1.ReadTimeout = 100


        'SerialPort1.ReceivedBytesThreshold = 100
        SerialPort1.DiscardNull = True

        Try
            SerialPort1.Open()
            'rs232Timer.Interval = 1
            'rs232Timer.Start()
            vcapMin1 = 9999
            vcapMax1 = 0
            vcapMin2 = 9999
            vcapMax2 = 0
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End If
    ' ONCE PORT IS INITIALISED, RECEIPT OF DATA KICKS EVERYTHING OFF ...
End Sub

' This is the method to run when the rs232Timer is raised.
'Private Sub TimerEventProcessor(ByVal myObject As Object, ByVal myEventArgs As EventArgs) Handles rs232Timer.Tick
Private Sub SerialPort_DataReceived(sender As Object, e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived
    'rs232Timer.Stop()
    Do
        Try
            Dim Incoming As String = SerialPort1.ReadLine()
            If Incoming Is Nothing Then
                Exit Do
            Else
                Dim packet As String = Incoming
                Incoming = Nothing
                If (RequestCmd > 0) Then
                    If SerialPort1.IsOpen = True Then
                        SerialPort1.Write(Chr(RequestCmd))
                        System.Diagnostics.Debug.WriteLine("CMD: " & RequestCmd)
                        RequestCmd = 0
                    Else
                        System.Diagnostics.Debug.WriteLine("CMD: " & RequestCmd)
                    End If
                End If
                Me.BeginInvoke(New StringSubPointer(AddressOf ProcessData), packet)

            End If
        Catch ex As Exception
            Exit Do
        End Try
    Loop

    'rs232Timer.Start()
End Sub

Private Sub ProcessData(ByVal Buffer As String)
    Dim rawLine As String
    If Buffer.Length > 0 Then
        Select Case Mid(Buffer, 1, 1)
            Case "P" 'PON registers
                processPonRegs(Buffer)
                If (Mid(Buffer, 2, 1) = "1") Then
                    If (p1Active = True) Then
                        rawLine = Mid(Buffer, 1, 2) & " " & Mid(Buffer, 3, 16) & " " & Mid(Buffer, 19, 16) & " " & Mid(Buffer, 35, 16) & " " & Mid(Buffer, 51, 16)
                        storeRawDataLine(rawLine, 2)
                        rawLine = "   " & Mid(Buffer, 67, 16) & " " & Mid(Buffer, 83, 16) & " " & Mid(Buffer, 99, 16) & " " & Mid(Buffer, 115, 16)
                        storeRawDataLine(rawLine, 3)
                    End If
                Else
                    If (p2Active = True) Then
                        rawLine = Mid(Buffer, 1, 2) & " " & Mid(Buffer, 3, 16) & " " & Mid(Buffer, 19, 16) & " " & Mid(Buffer, 35, 16) & " " & Mid(Buffer, 51, 16)
                        storeRawDataLine(rawLine, 4)
                        rawLine = "   " & Mid(Buffer, 67, 16) & " " & Mid(Buffer, 83, 16) & " " & Mid(Buffer, 99, 16) & " " & Mid(Buffer, 115, 16)
                        storeRawDataLine(rawLine, 5)
                    End If
                End If
            Case "T" 'Trace Buffer Data
                'saveTraceBufferLine(Buffer, TBConfiguration)
                My.Computer.FileSystem.WriteAllText("TraceBuffer.txt", Buffer, True)
                rawLine = Buffer
                filedata = Buffer
                storeRawDataLine(rawLine, 6)
                TBLine = TBLine + 1
            Case Else 'Status data
                If Mid(Buffer, 4, 1) = "S" Then
                    processStatus(Buffer)
                    rawLine = Mid(Buffer, 4, 1) & "  " & Mid(Buffer, 5, 16) & " " & Mid(Buffer, 21, 16) & " " & Mid(Buffer, 37, 16) & " " & Mid(Buffer, 53, 16)
                    storeRawDataLine(rawLine, 1)
                    displaySerialNumber()
                    displayFirmware()
                    displayAlarms()
                    increment(nscans1)
                    ProgressBar1.Value = nscans1
                End If
        End Select
    End If
End Sub