If it's possible to type text into your drop down list field, use OnObjectExit as your script trigger. The script can check the contents of the field and use If steps to determine what action to take. Don't use OnObjectModify if its possible to type text into the drop down's field as each keystroke will fire the OnObjectModify trigger. (OnObjectModify is a good choice for pop up menus, check box and radio button formatted fields.)
Thanks Phil...will give that a try.