intermittent disconnects

Hi,

We keep having T3/T4 errors with disconnects about every hour. We have replaced modem, splitters, cables and router. Called Cox to schedule an appointment, it was scheduled, waited at home and nobody showed. Called again to learn that Cox had canceled the scheduled appointment because when they probed our modem we happened to have a connection at that time. How do I convince Cox to send out a technician after scheduling one in a scenario like this?