My ethernet connection and wifi drop to less than 1mb from 60+ mbps regularly. I recently got a replacement modem from xfinity and the problem is still happening. Along with the 1mbps, the connection will drop 20-30 times a day.
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.
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 chat with them using the Internet Support option at https://support.xfinity.com/chat. If they can't fix the problem remotely, insist they send a tech out to identify the cause and correct it. Loss of Internet service is often due to a poor connection between your equipment and Comcast's network, usually in or near your home.
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.
Were you able to find a cause for your speed issues? If not, I can take a deeper look into this. Can you please post your speeds after reviewing the links provided by@BruceW?