一个向左无缝滚动的图片 滚动一段时间为什么自动停止了？

Question

<div id="demo" style="width:900px; overflow:hidden; height:110px;">
    <div id="demo1">
    <table width="90%" border="0" cellspacing="0" cellpadding="0" class="gd">
     <tr>
      <td><img src="pro2/2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/7-2-small.jpg" border="0" width="145" height="100"></td>
      <td><img src="pro2/8-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/9-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/10-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/11-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/12-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/12-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/14-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/17-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/17--2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/7-2-small.jpg" border="0" width="145" height="100"></td>
      <td><img src="pro2/8-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/9-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/10-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/11-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/12-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/12-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/14-2-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/17-small.jpg" width="145" height="100" border="0"></td>
      <td><img src="pro2/17--2-small.jpg" width="145" height="100" border="0"></td>
     </tr>
    </table>
</div>
<div id="demo2"></div>
</div>
<script> 
var speed=1 
demo2.innerHTML=demo1.innerHTML 
demo.scrollLeft=demo.scrollWidth 
function Marquee(){ 
if(demo2.offsetWidth-demo.scrollLeft<=0) 
demo.scrollLeft-=demo2.offsetWidth 
else{ 
demo.scrollLeft++ 
} 
} 
var MyMar=setInterval(Marquee,speed) 
demo.onmouseover=function() {clearInterval(MyMar)} 
demo.onmouseout=function() {MyMar=setInterval(Marquee,speed)} 
</script>请高人赐教！