Intermittent problems can only be diagnosed when the system is not working and can be difficult to diagnose because you may never be sure you fixed the problem. There could be an issue with a loose connection or a failing control unit and a qualified technician will need to spend some time to sort it out once it acts up. It would help if you could analyze the systems operation to see if there was any consistency to when it quit working. If there is then the tech could duplicate the problem and repair the issue.