Flexslider issue on IE9

Flexslider issue on IE9

I've been testing this website in several browsers now:


However, for some odd reason, IE9 has a bug displaying the last slide (most of the time, thou sometimes it does it on other in-between slides) on pages such as this:


When you click the next button, the slideshow seems to go from beginning to end in a fast forward way and then it displays the last slide. On some computers, it quickly displays the last slide and then moves quickly to the first. This behaviour only happens on IE9. It works perfectly well on IE8 and all other browsers. Any ideas as to why it is doing this?

This is the code that am using:

$(window).load(function() {
$('#portfolio-slider').flexslider({
animation: "slide",  
slideDirection: "horizontal",  
slideshow: false,              
slideshowSpeed: 3500,      
animationDuration: 500,
directionNav: true, 
controlNav: false          
});
});

Thanks!