Answer for Combo modem router

I would first run your problem by Comcast support to see if there are any issues they can detect with their service. Beyond that, you should troubleshoot your network to try to isolate where the issues are. I wrote an article not long ago that can help you with this. Read it and let us know if you can nail down the trouble.
https://marcelbrown.com/2015/08/13/is-your-internet-speed-not-what-it-should-be-how-to-test-and-isolate-the-problem/