It isn't that Cox is terrible, it's that people complain without taking the correct steps to rectify the issue. I have had Cox since I bought my home in 1993. It's been reliable, and and when they offered internet, I got it as well. I currently have 500/50 internet, and it's as fast and reliable as I could wish for. 
That being said, I maintain my indoor cable wiring, and replace barrel connectors and splitters every few years. I ensure that indoor cables are not bent, (The center wire carries an RF signal, and a bent wire can cause it to reflect backwards and cause issues). If your modem is losing sync, start by replacing indoor cables, (device to wall, barrel connectors, (on the wall plates)), and splitters. Ensure your modem/routers are up to date, if you rent from Cox, take it to the solution store and swap for a new device. If you have purchased your equipment, make sure it's on the approved list for the speed you are paying for. Too many times when I worked for Cox, I would get calls where someone's device was GRANDFATHERED, and set to a slower speed, requiring replacement so people could get the speeds they are paying for. https://www.cox.com/residential/support/cox-certified-cable-modems.html
Do you have devices connected via ethernet? Are they showing the same issues? If not you may have problems with WiFi interference in the home. Moving your router to a central location, raised as high as you can, and ensure it's not under or behind ANYTHING. It HAS to be in the open.
If all this does nothing, you may have issues with the service coming up to your home, call Cox tech support, request escalation to CAG, and provide them ALL the details you can. equipment, steps you have taken to rectify the issue, the fact it's happening on all your devices.
