Schedule a technician out(or 2 or 3) to document the problem. Technicians cost Cox money, so if you make them spend enough they will eventually send out a in-house technician or even a field supervisor to investigate. If not, try filing a informal FCC complaint. That should at least get you in touch with the right people.
Another thing you can do is ask for Tier 2(AKA CAG) and see if they have the tools to look at other modems on your street and see if they are also getting escalating T3 errors. Getting to Tier 2 is hard these days I hear. They may require you to have Cox Complete Care(CCC) because you have a retail modem but you DON'T want to talk to CCC. They are outsourced and terrible IMO. Instead try to get to Customer Advocacy Group(CAG). Good luck.
BTW, Are you in AZ? I hear some people are having issues in parts of that state. ::edit:: I am assuming Phoenix by your username.