I’m using the standard Foundation Top Bar. On some of my pages, it it very jumpy. Very occasionally, I also get all my menu buttons disappearing - leaving only the top bar itself.
It mainly occurs on the following page, but I have seen it on others as well.
If you load the page and then scroll down a bit and then back up, and repeatably do this you will see what I am attempting to describe. Sometimes, the menu buttons don’t appear at all!