if u dont get sol yet plz try this..

when u populate the combo
may be u write alert or something
after xmlHttp.send(params);
and before checking ready state
first check the state of ur xmlhttp and then write whatever u want to write
u can check ready state with

xmlHttp.onreadystatechange=stateChanged;



where stateChanged is function..


function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById("alertdiv").innerHTML=xmlHttp.responseText
//here is ur operation
xmlHttp=null
}
}
i hope this will work
nisha

any chance you are "using" something before it's been "declared" ??? this can cause weird things to happen on slower machines before data/files have been cached the first time.