javascript 中的文字向上滚动效果在鼠标移到上面再离开时文字滚动就变了请各位高手指点

<script language="javascript" type="text/javascript">
<!--
var demo = document.getElementById("demo");
var demo1 = document.getElementById("demo1");
var demo2 = document.getElementById("demo2");
var speed=30;    //滚动速度值，值越大速度越慢
demo2.innerHTML = demo1.innerHTML    //克隆demo2为demo1
function Marquee(){
    if(demo2.offsetTop-demo.scrollTop<=0)    //当滚动至demo1与demo2交界时
        demo.scrollTop-=demo1.offsetHeight    //demo跳到最顶端
    else{
        demo.scrollTop++
    }
}
var MyMar = setInterval(Marquee,speed);        //设置定时器
demo.onmouseover = function(){clearInterval(MyMar)}    //鼠标经过时清除定时器达到滚动停止的目的
demo.onmouseout = function(){MyMar = setInterval(Marquee,speed)}    //鼠标移开时重设定时器
-->
                                            </script>