Having problem with movable popup

I have a popup and I want to make it movable.  I have almost got it, but it move in a funky way sometimes (the popup will move but the mouse pointer is not over the popup).

XAML:

<Popup Name="Part_Popup" IsOpen="{Binding ElementName=PART_TogglePlayList, Path=IsChecked}" StaysOpen="True"><StackPanel Orientation="Vertical" ><Rectangle x:Name="theRect" Height="15" Fill="Gray" ></Rectangle><ItemsControl  x:Name="Part_PlayList" ItemsSource="{Binding songList}" Background="Wheat" HorizontalContentAlignment="Left" HorizontalAlignment="Stretch"  
                                         ToolTip="Click to see playing song list" 
                                         ItemTemplate="{Binding SongListDisplayDataTemplate}"
                                         ScrollViewer.CanContentScroll="True" 
                                         ScrollViewer.VerticalScrollBarVisibility="Auto"></ItemsControl></StackPanel></Popup>

Here are the three methods used in code behind:

#Region "Popup movement"

        Private Sub theThumbMouseLeftBUttonDown(sender As Object, e As MouseButtonEventArgs)
            mouseIsDown = True
            Dim pop As Rectangle = CType(sender, Rectangle)
            oldMousePosition = pop.PointToScreen(e.GetPosition(pop))
            pop.CaptureMouse()
        End Sub

        Private Sub theThumbMouseMove(sender As Object, e As MouseEventArgs)
            If Not mouseIsDown Then
                Return
            End If

            Dim pop As Rectangle = CType(sender, Rectangle)
            'Dim pop As Popup = thePopup
            Dim newMousePosition = pop.PointToScreen(e.GetPosition(pop))
            Dim offset = newMousePosition - oldMousePosition
            oldMousePosition = newMousePosition
            pop.Tag.HorizontalOffset += offset.X
            pop.Tag.VerticalOffset += offset.Y

        End Sub

        Private Sub theThumbMouseLeftButtonUp(sender As Object, e As MouseButtonEventArgs)
            Dim pop As Rectangle = CType(sender, Rectangle)
            mouseIsDown = False
            pop.ReleaseMouseCapture()
        End Sub
#End Region

Anyone see my problem

Lloyd Sheen