TL;DR Switch if you can, it's not going to get better.
Cox won't admit it, but it's mostly due to over subscribing their nodes and pocketing the revenue instead of putting the money back into their infrastructure to support their clients. It's simple corporate greed. In a lot of places they have a monopoly on broadband and with no competition, they have no reason to upgrade.
If you have another option, you should do that. I also have gigablast and am thinking about switching to CenturyLink 50Mbps fiber, because I'd rather have stability than "promised" speeds that can't be delivered.
Also, I'm sure the moderators will chime in with "send a message to cox help so we can look into it", but they don't care. They're just here to give the customer false hope. It's just a smoke screen.