WPF Contextmenu with gridcontrol

Question

One Contextmenu in my application has Grid controls ( with buttons ) as menuitems.

<RichTextBox.ContextMenu>
    <ContextMenu  Width="280">
        <Grid Width="240" Height="65">
            [...]
        </Grid>
    </Contextmenu>
</RichTextBox.ContextMenu>

The Problem is when I first open the ContextMenu and move the mouse over it, all Itmes (grid controls) are moved a few pixels the the left... maybe a layout problem

When the Width of the Contextmenu is set to "Auto" this doesn't happen, but then the Contextmenu is much bigger than it should be.

This image shows the problem

However, the Menu works. So it is just a esthetic issue, but it would be nice if it would be drawn correctly.

Can anyone help me with this problem.