contextmenu is flickering in windows xp

I have a contextmenu for a user control: xControl, the xControl will update every second based on some realtime data, so the context menu is blinking. but that only happens in xp. in windows 7, the context menu works fine:

<c:ThreeWayValveControl Canvas.Top="8" Canvas.Left="80" Width="40" ValvePosition="{Binding ValvePosition, ElementName=ControlRoot}" LineID="{Binding LineID, ElementName=ControlRoot}"><c:ThreeWayValveControl.ContextMenu><ContextMenu><MenuItem Header="Block" Command="{Binding BlockCmd }" CommandParameter="{Binding PlacementTarget.LineID, RelativeSource={RelativeSource AncestorType=ContextMenu}}" /><MenuItem Header="Supply" Command="{Binding SupplyCmd}" CommandParameter="{Binding PlacementTarget.LineID, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/><MenuItem Header="Vent" Command="{Binding VentCmd}" CommandParameter="{Binding PlacementTarget.LineID, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/></ContextMenu></c:ThreeWayValveControl.ContextMenu></c:ThreeWayValveControl>

Qun Li