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Same issue. I've had great internet for the past 11 months. Recently, past 2-3 weeks, I drop internet every 20-30 minutes. I have the C7800, too. Verified the device is working optimally. I've had a technician out that said it's because my router is off a splitter, moved to a direct input from the house and I still have the same issue.
Start here;
https://forums.xfinity.com/t5/Your-Home-Network/Connection-Troubleshooting-Tips/m-p/1253575#M94474
Post the requested info from here;