Are you connecting via Wifi or Ethernet?
If you want to troubleshoot this yourself, please see Connection Troubleshooting Tips. If you still need help, please provide Information Requested for Connection-Related Posts. Problems with Internet service are often due to poor connections between your equipment and Comcast's network, usually in or near your home.
If you are using Wifi it's best to switch to an Ethernet cable connection if possible while measuring speed and checking the reliability of your connection to Comcast.
If you can't find the problem or you'd rather have Comcast do the troubleshooting, call them at the phone number on your bill or 1-800-Comcast, or or use one of the options on https://www.xfinity.com/support/contact-us/. If they can't fix the problem remotely, insist they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast rental device, or anything outside your home, you shouldn't be charged.