Header items not keeping position

Hi friends I am new to this forum. Apologies if this has been covered in another thread. I have a header on my website which then sits on top of a menu. The header is one pixel above the menu to create a nice crisp white line differentiating between the two elements. No matter what I do to synch the header/ menu position on all pages the position randomly changes for 1-2-3 pixels depending on which way the wind is blowing. Perhaps an element of OCD but one pixel gap should be uniformly just that. Any advice would be gratefully appreciated thanks in advance.