I had the same issues. Finally got someone to come out that wasnt a contractor and has been with Cox a while and knew what he was doing. Come to find out, the Panoramic Wifi Modems are not all Equal. he went through 5 modems in his truck before he got one to work. he even let me hook my laptop direct up and verify gig speeds before he left.
I then hooked up to my network and gig speeds went away. Went back to the modem direct connect and gig was still there so I knew then it was a Me issue.. Found out my Wifi Router even though it supports Gigabit, couldn't support gig speeds, plus firewall, usb device and 20+ wifi devices. the CPU couldn't keep up. So got a new Router Problem solved.
This info may or may not help you.. Took me 3 months of techs to get the one guy who literally went through 5 modems till he found one that worked. (Also Later on I ended up buying my own so I wouldnt have to rent)