求一段滚动显示的js代码

比如说我有9行字
每次显示3行 停顿1秒后 往上滚动 再显示4.5.6三行 再停顿 再滚动显示7.8.9 现在的问题是 显示完4.5.6后滚动到7.8.9的时候 他会滚动到一半 又显示1.2.3行了 7.8.9的时间不够
<script type="text/javascript">
<!--
var marqueesHeight=60;
var stopscroll=false;
var scrollElem = document.getElementById("scrollTable");
with(scrollElem){
style.width = 200;
style.height = marqueesHeight;
style.overflow = "hidden";
noWrap = true;
//onmouseover=new Function("stopscroll=true");
//onmouseout=new Function("stopscroll=false");
}

scrollElem.onmouseover=new Function("stopscroll=true");
scrollElem.onmouseout=new Function("stopscroll=false");

var preTop=0;
var currentTop=marqueesHeight;
var stoptime=0;
initSrollText();

function initSrollText(){
scrollElem.scrollTop=0;
setInterval("scrollUp()",20);
}