var hoverInterval;

function doStuff_plus(){ doStuff(1); }
function doStuff_minus(){ doStuff(-1); }

function doStuff(hm) {
    
    var h = $('.thumb_slider div').height();
    //alert(h);
    var top = $('.thumb_slider div').css('top');
        
        top = top.replace('px', '');
        top = parseInt(top);        
    
    
        top += hm;
        
    var r =    h + top - 405;
    
    if(top<0 && r > 0)
    {
        $('.thumb_slider div').css({'top':top+'px'});
    }
}

$(function() {
           hoverInterval = setInterval(doStuff_minus,1);

    );
});