Ajax Post & IE8

I have an Ajax script that runs on IE9 & FF but not IE8...well it runs but there is some sort of memory leak so the script runs slower and slower with each consecutive click because the code being processed is not being cleared each time, or something. I found some articles online and tried them but with no luck. How do I stop the cycle?

 if (window.XMLHttpRequest){xmlhttp=new XMLHttpRequest(); }
        // code for IE
        else if (window.ActiveXObject){xmlhttp=new ActiveXObject("Microsoft.XMLHTTP"); }
        if (xmlhttp!=null){
            
            $.ajax({type: "POST", url:"/load-content/", global: false, data:({img:showcase, clicked:click_name, off:off_name,menuv:aname}), async:true, dataType: "html", success: function(xml) {
                   // format and output result  
               
                   nshow = jQuery("div p.p2").html(xml);
                
            xmlhttp= null;
            }
        });

        return false;
        }