Do the standard checking that Cox recommends, to verify all your connections are properly connected. If the issue persists, the intermittent pixelation could be caused by a weak signal. You'll need a tech visit to have your signal checked. If the tech finds a signal issue on Cox's side of the demarcation point (box where Cox's cables connect to your cables), repairs will be made at no cost to you. The tech may perform repairs or notify Cox for escalation to a line tech.
You could try contacting cox.help@cox.com to see if they can test your signal remotely, but I suspect they'll advise you to schedule a tech visit. They might schedule it for you if you confirm that's what you want done.