Primary menu lay-out changes unpredictable

Hallo, We use 8 buttons in the primary menu, plus the magnifying glass for the search engine. On a wide computer screen the display of the menu is fine. If we reduce the width of the browser frame to simulate a smaller screen (or we use a computer with a smaller screen) the last button and the magnifying glass are placed below the other part of the primary menu. Each in an "own row", so the primary menu triples actually in height. The original layout is lost. If we continue with "narrowing" the browser frame, the menu lay-out jumps to the lay-out where there is only a menu icon (3 horizontal bars) in the left and right top corner of the screen. This is the correct lay-out for smart phones etc. I can reduce the effect a little by reducing the menu with one knob, so there is less text in de primary menu bar. But actually we need the space for the additional knob and sufficient text-space for the other knobs. How can this be solved? Tanks