Nope, it's not normal. It means your modem is losing connection and then reconnecting.
Start here:troubleshooting guide. If you still need help, please post your equipment details (make/model of your modem/gateway/router) and a screenshot of your signal levels and your error/event log. Or if you aren't technically inclined, we can ask an employee to set you up with a service call.
I am not a Comcast employee, just a moderator. Pls observe Wheaton's Law.
