Visual Basic.NET - Problema con el Puerto Serie

Public Sub Busca_Puertos()

        MnPuerto.DropDownItems.Clear()

        For Each PuertoSerial As String In My.Computer.Ports.SerialPortNames

            MnPuerto.DropDownItems.Add(PuertoSerial)

        Next

End Sub

Private Sub btnConectar_Click(sender As System.Object, e As System.EventArgs) Handles btnConectar.Click

        Call CompPuerto()

        If btnConectar.Text = "Conectar" Then

            btnConectar.Text = "Desconectar"

            btnConectar.BackColor = Color.Lime

            TabControl.Enabled = True

            Recibe_Datos()

        Else

            btnConectar.Text = "Conectar"

            btnConectar.BackColor = Color.Red

            TabControl.Enabled = False

            Call Cierra_Puerto()

        End If

End Sub

Public Sub CompPuerto()

        Call Busca_Puertos()

        If Puerto.IsOpen = False Then

            If Puerto.PortName = "COM6" Or MnPuerto.DropDownItems.Count = 0 Then

                If MnPuerto.DropDownItems.Count >= 1 Then

                    MsgBox("SELECCIONES PUERTO DE COMUNICACION", MsgBoxStyle.Information + MsgBoxStyle.OkOnly, )

                    Exit Sub

                Else

                    MsgBox("NO HAY PUERTOS DISPONIBLES", MsgBoxStyle.OkOnly, )

                    Exit Sub

                End If

            Else

                Call Abre_Puerto()

            End If

        End If

End Sub

Public Sub Abre_Puerto()

        '   Puerto = My.Computer.Ports.OpenSerialPort("COM6")

        Puerto.BaudRate = 4800

        Puerto.StopBits = StopBits.One

        Puerto.DataBits = 8

        Puerto.Handshake = Handshake.None

        Puerto.Parity = Parity.None

        Puerto.ReadTimeout = 5000

        Try

            Puerto.Open()

            If Puerto.IsOpen = False Then

                Exit Sub

            Else

                ' Añadir el codigo

            End If

        Catch ex As TimeoutException

            MsgBox("Fallo de Comunicacion", MsgBoxStyle.Information + MsgBoxStyle.OkOnly, )

        End Try

End Sub

 Public Sub Recibe_Datos()

        Try

            Do

                Dim Dato As String = Puerto.ReadLine

                If Dato Is Nothing Then

                    MsgBox("No se reciben datos", MsgBoxStyle.Information + MsgBoxStyle.OkOnly, )

                    Exit Do

                Else

                    '  TextBox1.Text += Dato & vbCrLf

                    clsMonitor.txtMonitor.Text += Dato & vbCrLf

                  '  Application.Run()

                End If

            Loop

        Catch ex As InvalidOperationException

            Exit Sub

        End Try

    End Sub

Public Sub Cierra_Puerto()

        Puerto.Close()

End Sub