They say a pictures worth a thousand words, so here's what I'm facing non stop.
When I first called Comcast, they said it was because my data plan was too low, so I upgraded to gigabit. Then when I had issues, they said it was my Netgear Nighthawk modem that was causing the problem. I swapped to their modem and same thing. I've since swapped the co axial cable, the ethernet cable to my desktop and same thing. I tried using my laptop and same thing.
Please see Internet Troubleshooting Tips. If you still need help, please post your modem or gateway's signal information, especially downstream power levels and SNR, upstream power levels, and error log.
What is the model number of your modem?