WPF Menu item "_" disappear on mouse over

Question

Hi,

I have implemented a Menu with hot keys functionality. Following is the XAML i used,

<Menu IsMainMenu="True" 
              x:Name="mainMenu" 
              Background="#FFACC9F7" 
              Height="20" 
              Margin="1,6,-1,0" 
              VerticalAlignment="Top">

            <MenuItem x:Name="fileMenuItem" Header="_File" Margin="10,0,0,0" >
                <MenuItem Header="_New" InputGestureText="Ctrl+N" IsCheckable="True" >
                <MenuItem Header="_Edit..." InputGestureText="Ctrl+E" /></MenuItem>
            </MenuItem>
        </Menu>

When i press on the Alt key, the Under score appear in _File menu item and once you click on this and move the mouse over the Sub menu items(_New/_Edit), the Underscores disappear in all menu items. (The Underscore remains intact when you navigate using only keys.)

How can i stop the Underscore from disappearing on mouse over?

Thanks.