If you roll over the About Us button the links in the drop down menu shift a pixel on the hover. I'm think some kind of of child-first is needed but I've spent hours fiddling with this. This is a jQuery WordPress navigation plugin. cermedcorp.com/
From what I can see, for those submenus, you give them an additional border on top and bottom once the mouse is on it. But in order to not make it shift, that border should also be there when you don't hover on it, so I would give it a white border in its normal state and a grey one on mouseover.
cermedcorp.com/