求div里js滚动遇到的问题. 代码是文字滚动从下向上滚动.

<div id="latest_site_news" class="colbox">
<h3> <!-- {if $site_news.info.page_count gt 1} --> <em class="more"> <a href="{url src=index.php?app=article&act=site_news}">{$lang.more} </a> </em> <!-- {/if} -->{$lang.site_news} </h3>
<div id="news_scroll_bar" style="overflow:hidden; height:180px;">
<div id="news_scroll_items" class="colboxcontent">
<ul>
<!-- {foreach from=$site_news.data item=article} -->
<li> <a href="{url src=index.php?app=article&act=site_news&id=$article.article_id}">{$article.title|escape} </a> </li>
<!-- {/foreach} -->
</ul>
</div>
<div id="news_scroll_replaces" class="colbtm"> </div>
</div>
</div>
<script language="javascript">
var news_scroll_bar = document.getElementById("news_scroll_bar");
var news_scroll_items = document.getElementById("news_scroll_items");
var news_scroll_replaces = document.getElementById("news_scroll_replaces");
var speed = 70;//滚动速度
if(isFirefox=navigator.userAgent.indexOf("Firefox")>0){
var rows = 100;//每行高度
}

var stim = 10; //停留时间倍数 * speed
var stop = 10; //初始化值，不管

news_scroll_replaces.innerHTML=news_scroll_items.innerHTML
function Marquees(){

if(news_scroll_replaces.offsetTop-news_scroll_bar.scrollTop <=0)
news_scroll_bar.scrollTop-=news_scroll_items.offsetHeight
else{
news_scroll_bar.scrollTop++
}
}
var MyMars=setInterval(Marquees,speed)
news_scroll_bar.onmouseover=function() {clearInterval(MyMars)}
news_scroll_bar.onmouseout=function() {MyMars=setInterval(Marquees,speed)}
</script>


目前这段代码,走一走就停了. 而且速度有点慢. 求高人帮忙解决下.