If you’re using Chrome and seeing your admin menu items jump around or hide, it’s not WordPress – it’s a known Chrome bug and it is going to be fixed in a soon-to-be released version. But in the meantime, here’s how to fix it.
In Chrome open a new tab and paste or type this in the address bar:
Enable the first item in the list you see and make sure the second item is not enabled. Then restart Chrome and the problem should be fixed.