If you haven't I'd do a full inspection of external connections & then to the inside to where the coax connects to the modem. I've had my line replaced twice in 10 years. Once squirrels got into it where they were running down the line and jumping to a tree near the aerial connection block. They chewed the insulation off the coax. The other time it was a corroded external splitter caused by condensation inside the junction box. Occasionally they will have a channel that gets to strong and need to filter it down to decrease interference as well. If your equipment is in good state you should be set.
It's most likely the filters. When I had an issue like this a while back, a Cox tech would come out, test the line and add a filter. Then after a while, the problem would come back. Another Cox tech would come out and say the filter should not be there. Rinse and repeat a few times and I finally asked for a supervisor to drive out and look at the issue. Turned out it was a problem at the pedestal.
Complete incompetence.
