When you are experiencing a signal issue, it could be something between you and the provider including any and all RF cabling or the equipment on either end. The equipment on the Cox end is not likely at issue otherwise there would be widespread localized reports of problems. This leaves the modem with respect to an equipment issue, while not likely, it's possible. I would not suggest buying a new modem to test this theory unless the modem you're using is otherwise obsolete in some way. I see in your signal levels, OFDM support indicated so I would say based on that your modem is probably not functionally obsolete.