TreeView bound to XmlDataProvider not updated when XmlNode modified

My application is basically a specialized XML editor. I display the XmlDocument in a TreeView declared as:

<TreeView Name="asifTreeView"
                ItemTemplateSelector="{StaticResource treeItemTemplateSelector}"
                SelectedItemChanged="asifTreeView_SelectedItemChanged"
                PreviewTextInput="asifTreeView_PreviewTextInput"
                ItemsSource="{Binding Mode=TwoWay, XPath=*}"
                Loaded="asifTreeView_Loaded"
                Margin="5"/>

The DataContext is an XmlDataProvider that is created by this code:

            Data = new XmlDataProvider();
            Data.XPath = "*";
            Data.DataChanged += Data_DataChanged;

For XmlElement nodes with a single XmlText node (which hold the data for my application), treeItemTemplateSelector returns this HierarchicalDataTemplate:

<HierarchicalDataTemplate x:Key="textNodeTemplate"><StackPanel Orientation="Horizontal"
              ToolTip="{Binding Converter={StaticResource xmlNodeToToolTipConverter}}"
              ContextMenu="{Binding Converter={StaticResource xmlNodeToContextMenuConverter}}"><TextBlock Text="{Binding Path=Name}"/><TextBox Text="{Binding Path=FirstChild.Data, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/></StackPanel></HierarchicalDataTemplate>

In my view model I track the changes to the content of the TextBox by handling the NodeChanged event:

                doc.NodeChanged += NodeChangedHandler;

private void NodeChangedHandler(Object src, XmlNodeChangedEventArgs args)
{
    UndoStack.Push(new EditorOperation(args));
}

Then I implement 'Undo' by doing:

        public void Perform_Undo()
        {
            if (UndoStack.Count > 0)
            {
                EditorOperation changeData = UndoStack.Pop();

                if (changeData.Action == XmlNodeChangedAction.Change)
                {
                    changeData.Node.Value = changeData.OldData;
                 }
            }
       }

I can see the XmlNodeChanged event fire and I record the change in my UndoStack. When the Perform_Undo() function is called I see the data in the XmlDocument change, but the TextBox in the TreeViewItem is not.

I had an earlier problem with data going the other way (changes made in the TextBox not being propogated to the XmlDocument) but that was fixed by adding UpdateSourceTrigger=PropertyChanged to the ItemTemplate in the HierarchicalDataTemplate.

Can anyone explain why the binding does not seem to be working in this case? How do I fix it?

Thanks,

Bill