Site navigation jumping to middle of header when browser re-sizes

Something strange is happening with the site navigation on browser re-size. The menu jumps to the middle of the header and therefore goes behind the logo when the browser is resized manually. On some laptops, it's showing in the middle of the header too. I tried to change the settings of the menu (for example I set it to show above the header) but it was the same. It needs to stay in the correct place which is right above the secondary navigation and under the logo. I can show you screen grabs if you need that? Thanks for your help.