Using JavaScript to Query List

I am querying a list using JavaScript, from time to time the list items are retrieved, however, there are times that it returns an access denied error.

var web = clientContext.get_web();

var list = clientContext.get_web().get_lists().getByTitle("Employee Hours Profile");

var camlQuery = new SP.CamlQuery();

camlQuery.set_viewXml('<View>' +

     '<Query><Where><Eq><FieldRef Name='Title'/><Value Type='Text'>[ItemTitle]</Value></Eq></Where></Query>' +

          '<ViewFields>' +

               '<FieldRef Name='Title' />' +

          '</ViewFields>' +

     '<RowLimit>1</RowLimit>' +

'</View>');

var listItems = list.getItems(camlQuery);

clientContext.load(listItems);

clientContext.executeQueryAsync(onSuccess, onFail);

function onSuccess() {

     var listItemEnumerator = listItems.getEnumerator();

     while (listItemEnumerator.moveNext()) {

          var listItem = listItemEnumerator.get_current();

          var title = listItem.get_item('Title');

     }

     NWF$("#" + varTitle).val(title);

}

function onFail(sender, args) {

     alert("Error");

}