Ajax/Prototype onComplete fires before response returned in

Simple ajax call seems to have some issues in Firefox. The
“onComplete:” is
called BEFORE the response is returned by the call. Is there a coding
issue
or a work around?

var ajax = new Ajax.Request( url, {method: ‘post’, parameters: params,
onComplete: evalInfo });

function evalInfo( request )
{
// do stuff with request
}

Should I have a timer that checks the request state before exec the
evalInfo?

Thanks