Basically slow speed, intermittent connection or no internet access occurs usually out of cable signal level, out of acceptable range and it will be resolved after the power level are adjusted by the service provider. So for this we need to check the cable signal level to confirm what causing this issue to proceed further. Please use the link below to check the cable signal level on the interface.

http://arris.force.com/consumers/articles/General_FAQs/SB8200-Cable-Signal-Levels

It's nice to know that Cox (ISP) has checked the cable lines are good, the slow speed that you have been experiencing has been happening for 2 months could you let us know if it is happening when the modem is connected directly to the computer /laptop or router? 

If it is a direct connection, then check if the lights on the modem Power/Receive/Send and online are solid or flashing also check the cable signal levels 

http://arris.force.com/consumers/articles/General_FAQs/SB8200-Cable-Signal-Levels

If it is connected directly to the router then it could be a Wi-Fi issue in this case contact your router provider and change the Wi-Fi channels