If you can access your DMarc, I'd get in there and disconnect everything that isn't being used OR if possible, just bypass everything and connect from Cox's cable (the one from the street to your DMarc) to your modem with as little connections as possible and see what your levels look like. As a reference, when I bypass everything in my DMarc and just have 2 pieces of coax between my modem and the street, my levels are about 10dBmv, but that's not optimal so I put in a 2 way splitter to attenuate it down.
Also, if you decide to run your own cable for troublshooting, make sure it's quality RG6.
Keep it mind, it's Cox's stance with everything wrong with their service to blame the end user, so if you can bypass everything from the DMarc to your modem with new cabling, it will give you better ammo to put the blame back on them.
Also, FYI, the cable from the street to your DMarc is their cable and they should replace it if it's bad. Doing so yourself could cause more problems in the long run, because it's a direct burial cable designed to withstand water that will build up in the conduit it's pulled through.
