It looks to me like you just need to get the latest position value before applying it...
$(document).ready(function(jQuery){
var bgimg = $('.stripe-style-2').css('backgroundImage');
$(window).scroll(function () {
var bgpos = $('.stripe-style-2').css('background-position');
$('.fix #main-nav').css('background-image', bgimg);
$('.fix #main-nav').css('background-position', bgpos);
});
});
You were previously getting it only once and that variable would never change, despite the background position changing.