DataGrid Bound to XML not Refreshing

I have a DataGrid bound to an XmlDataSource.  Here is my (partial) XML:

<Settings xmlns=""><Profiles xmlns="" ActiveProfile="0"><Profile Id="0" Desc="- none -" Port="0" Baud="0" DataBits="0" Parity="0" StopBits="0" CharDelay="0" ReadTimeout="0" ProcInterval="0" SetInterval="0" DecInterval="0" ChartInterval="0" SaveInterval="0"><PIDs><PID Address="1" SetPoint="one" ProcVal="one" /><PID Address="2" SetPoint="two" ProcVal="two" /><PID Address="3" SetPoint="three" ProcVal="three" /></PIDs></Profile><Profile Id="1" Desc="Test Profile 1" Port="1" Baud="19200" DataBits="7" Parity="0" StopBits="3" CharDelay="1" ReadTimeout="100" ProcInterval="100" SetInterval="0" DecInterval="0" ChartInterval="0" SaveInterval="0"><PIDs><PID Address="" Name="" /></PIDs>
</Profile>
    ...
    ...
</Profiles>

I have a ComboBox bound to the Profiles, so when I select a new item, it updates the new Profile's Id attribute value into the parent node's ActiveProfile attribute.

Here is XmlDataProvider XAML:

<Window.Resources><XmlDataProvider x:Name="PIDData" x:Key="PIDData" Source ="Settings.xml" XPath="Settings/Profiles"/>

Here is DataGrid XAML:

<DataGrid AutoGenerateColumns="False" Height="201" HorizontalAlignment="Left" Margin="233,137,0,0" Name="DataGrid1" VerticalAlignment="Top" Width="444"
          ItemsSource="{Binding Source={StaticResource PIDData}, XPath=Profile[@Id\=../@ActiveProfile]/PIDs/PID}"><DataGrid.Columns><DataGridTextColumn Header="Address" Binding="{Binding XPath=@Address}"/><DataGridTextColumn Header="Set Point" Binding="{Binding XPath=@SetPoint}"/><DataGridTextColumn Header="Proc Val" Binding="{Binding XPath=@ProcVal}"/></DataGrid.Columns></DataGrid>

The XPath in the ItemsSource, which refers to the parent's ActiveProfile seems to work fine for my other controls which use the same type of binding.

When I select another item in the Profiles ComboBox, everything works as expected, except the DataGrid does not refresh.  If I have a different list of data items in a different XML node, it does not display, only the original list of items in the DataGrid remains.

All of my binding is done via XAML, none in code-behind.  Is there a way to cause the DataContext to refresh when I select a different ComboBox item?  I don't mind doing that in code-behind if that's what it takes (although it seems like it should work just from XAML).

Thanks...

Ron Mittelman