求教，滚动只执行了一次

<script type="text/javascript">
var speed=10; //数字越大速度越慢
var tab=document.getElementById("demo");
var tab1=document.getElementById("demo1");
var tab2=document.getElementById("demo2");
tab2.innerHTML=tab1.innerHTML;
function Marquee(){
if(tab2.offsetWidth-tab.scrollLeft<=0)
tab.scrollLeft-=tab1.offsetWidth;
else{
tab.scrollLeft++;
}
};
var MyMar = setInterval(Marquee,speed);
tab.onmouseover=function(){clearInterval(MyMar);};
tab.onmouseout=function(){MyMar=setInterval(Marquee,speed)};
</script>