Digital Tiling, or 'pixilation', is a normal occurrence NOW and THEN, it's the picture updating itself. If it's more common, then it's could either be cause by software on older boxes, or a signal issue. Signal issues can be anything from a bad signal to interference cause by a signal getting INTO the cable.
How to troubleshoot -
Start by replacing any splitters, and barrel connecters, (Barrel connecters are usually found in the cable plates on the walls), and replacing any cable wires from the barrel connecter on the wall plate, to the cable box, as well as modems. Once you have replaced all the cables and connectors you can reach, it's time for a tech to come out. Older boxes should be replaced ONLY if they are acting up. If they work in one room, not the other, focus on the ones not working. If it's happening to ALL the boxes in the home, it could still be only one box, as some systems have the main box providing the tuner for the other boxes, so focus on the MAIN cable box in that instance.
Cox tech support USED to be trained techs, now it's "Customer service" following a monkey script called solution center. It's useless as many people have found out, and one of the reasons I am glad I am no longer there.