There must be something off on this whole menu. When you resize a webpage, the menutop are to be responsive.
In this template when you resize down the browser, the menu dont become correct responsive. The way you describe this to me, it seems you have to address every possible browzer size to make to menu look correct.
1055 dont lok ok - 
1280 dont look ok - its missing the last menu item called Kontakt which probably are wrapped under the image.

Here are two other examples on me just changing the browser size randomly where the menu looks bad:
