Start here: troubleshooting guide. Please post your equipment details (make/model of your modem/gateway/router) and a screenshot of your signal levels and your error/event log.
If it's a bad device, Comast should come back out and swap it free of charge.
You're probably going to get billed for the last service call, you should talk to an employee about a credit.
FYI: They owe you a $20 credit for every tech visit after the first for the same problem.You can also get a credit for your service from the time you first reported the issue until it is resolved. getting a credit for that.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
I can help further investigate the issues you are having with your internet. In order assist you, please send me a private message verifying your first and last name, and the full account number or phone number associated with your services.
To send a private message click on my name "ComcastChe", then click private message me.