[рони]

Black_Star,
примерно так должно быть...

$(function() {
//var $navigationTop = ... , $navigationSite и т.д.
    function changeNav() {
        var coord = $navigationTop.offset().top + $navigationTop.height();
        if ($navigationSite.hasClass("navMob") && $(window).scrollTop() < coord) {
            buttonNav.css({
                display: "none"
            });
            navBar.show();
            $navigationSite.toggleClass("navMob")
        } else if ($(window).scrollTop() > coord) {
            buttonNav.css({
                display: "block"
            });
            navBar.hide();
            $navigationSite.toggleClass("navMob")
        }
    }
    $(window).scroll($.throttle(changeNav, 200))
});