I'm not the best with networking but this is what I would do.
Do you have an old modem you can test a hardwired speed directly to one computer? I'm not the biggest fan of modem/router combos especially directly from a cable company. Also do you have any neighbors with the same service that are experiencing the same issues? This would at least rule out the modem/router combo as the problem.
If you don't have an old modem you can just run to bestbuy or whatever pick up a docsis 3.0 and give that a shot. Return it after and whatnot.
You can also try resetting your router entirely. Just go back to the basic default settings, just save your old ones to your desktop. See if some settings are causing the issue. But if directly plugged in there is still an issue I'd aim to test that modem/router. I've never had a cable company give me a modem as good as any standard Docsis 3.0 Motorola or whatever.
EDIT Now that I think of it there was a time where Comcasts DNS server was having issues and went down. Trying changing your DNS server to Google DNS
https://developers.google.com/speed/public-dns/?hl=en
That might be a long shot but it has been a problem several times in the past.