I Have encountered the same issue on the system in my home, Mics 7.1XC CI cards with TWC voice
Turned out to be a known issue with the Arris Touchstone TM508 modem i had. Im on my second one because of static & the modem refusing to make dial tone or constantly dropping and syncing
Also ive noticed with almost all eMTA's, they are susceptible to back feeding frequency's if they are on the same AC circuit as anything that draws a large current which could explain static or intermittent modem dropping, Took 3 weeks for TWC and myself to narrow conditions down to inside my house and the dehumidifier in the basement on the same circuit branch.