var timer

window.onload = scrolltop;
window.onunload = stoptimer;

function scrolltop() {
var menu = document.getElementById('header');
var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
menu.style.top = scrollTop;
timer = setTimeout("scrolltop()",10);
}
function stoptimer() {
clearTimeout(timer)
}