Every time a page reloads or has an element load later down the page the header goes all out of wack (see image). How can this be fixed?

I made the website with an old version of Themler and I love how the main menu "grows on scroll" It was part of the original template so I have no idea how to recreate it. See here:...

The header doesn't seem so stable. When I zoom out the browser the header become shorter than the screen. Is there a setting for that? It is now 100% width. Site is: lovemetender.se I dont have that...