Neil has the fastest internet tier that Cox offers, but he's still not getting a consistent 300 Mbps speed. Sometimes it's about 20% of what it should be. Leo says that a DOCSIS 3 modem is ideal, and it's also better if his cable modem doesn't also do Wi-Fi. He should be using a third party router.
Also, Leo recently retired his Airport Extreme due to inconsistent performance. He replaced it with an Asus router. Neil should completely disable the wireless functions of the cable modem and let the router handle all the Wi-Fi traffic. But he should remember that the published speeds are ideal conditions and will fluctuate according to traffic, congestion and other things. If he reboots the router and the modem, chances are, it's Cox.