Please try with below code.
<SCRIPT language=JavaScript1.2>
var speed=1
var currentpos=0,alt=1,curpos1=0,curpos2=-1
function initialize(){
startit()
}
function scrollwindow(){
if (document.all &&
!document.getElementById)
temp=document.body.scrollTop
else
temp=window.pageYOffset
if (alt==0)
alt=2
else
alt=1
if (alt==0)
curpos1=temp
else
curpos2=temp
if (curpos1!=curpos2){
if (document.all)
currentpos=document.body.scrollTop+speed
else
currentpos=window.pageYOffset+speed
window.scroll(0,currentpos)
}
else{
currentpos=0
window.scroll(0,currentpos)
}
}
function startit(){
setInterval("scrollwindow()",50)
}
window.onload=initialize
</SCRIPT>
Check here for more info :
Java Script for Auto-Scrolling