请教一个ajax使浏览器崩溃的问题

在ie7下很正常,但是在ie6进行异步操作的时候有时候会使浏览器崩溃.
怎么回事?异步代码如下：function pollServer() {
    createXMLHttpRequest();
        var url = "index.php?action=main&do=updatepage";
        xmlHttp.open("GET",url,true);
        xmlHttp.setRequestHeader("If-Modified-Since","0");
        xmlHttp.onreadystatechange = pollCallback;
        xmlHttp.send(null);
}
function pollCallback() {
    if (xmlHttp.readyState == 4) {
            if (xmlHttp.status == 200) {
                   var XMLarticles = xmlHttp.responseText;
                   var xmlDoc = new ActiveXObject("Msxml2.DOMDocument");
                   xmlDoc.loadXML(XMLarticles);
                   message = xmlDoc.getElementsByTagName("info");
                                         if (message.length != 0) {
                                         makeTable(message,xmlDoc);
                                                 setTimeout("pollServer()",5000);
                         }
            }
    }
}