I have the same issue since Friday (10/09/2020). I have problems accessing several domains (amazon.com, americanexpress.com, canvas.ub.ca). However, other domains have no problem (google.com, yahoo.com, youtube.com).
Every time I reset the X1 Gateway, they start working again..until 10 or 15 minutes then it'll suddenly stop working (connection timeout).
Call Xfinity customer service, and a technician come to check the signal on my home. He said the signal is strong. He suggested me to get a new modem (X1 Gateway).
Got a new modem, and the behavior is still the same (intermittent connectivity).
I called again the customer service and he escalated to more specialized team that will contact me.
This is so frustrating since I cannot work (my company uses AWS and I cannot connect to aws.amazon.com). And my kids had missed several classes last Friday.