I’m also having that jumping issue on refresh as well as navigating. The menu is also jumping. I’m re-doing a friend’s site using Xtreme. I don’t recall having this issue with Weaver II Pro. . http://www.pirategirlfitness.com/
I am not sure the below issue is what you mean, but @weaver, this is what I observe.
As you load the page, the menu items occupy a certain width. As soon as you hover on a menu item, the width expands a bit and stays expanded after you un hover. If you do that on every menu item, it will expand a bit more every time, Once you have hovered all of them the menu bar items now occupy a wider area.
Will keep looking, but have not figured it out yet
One thing to try, and really the only starting point I can think of – do you still get the jumping if you turn the arrows back on? My guess is that there is some CSS rule that IE11 is not quite interpreting correctly that involves hiding the menu bar down arrows.