T3 errors are usually caused by signal problems, and mostly with the upstream frequencies that the modem uses to talk to Cox.
What kind of modem do you have? Also, can you post the signal levels? You can usually find them at 192.168.100.1 . Besides that, I would check all cable connections from where the cable comes in from the street, to where it connects to your modem.
Also, was the internet professional installed or self installed?
