I think you're having issues because you've multiple items sliding up at a given time. check this jsfiddle for solution
javascript
$(function () {
$(".close-project").click(function () {
$(".flexslider").slideUp("slow");
$("#work-list ul li").removeClass("current");
});
$(".client").click(function () {
var project = this.id;
var project_id = '#' + project + '-project';
var elem = $(".flexslider:visible").length? $(".flexslider:visible") : $(".flexslider:first")
elem.slideUp('slow', function () {
$(project_id).slideDown('slow', function () {
$(project).addClass("current");
})
});
});
});