I'm sorry that I cannot give a specific answer but I can give some things to try.
First clear you temp internet files then go to a problem site. If it fails try a refresh.
Domain Name Servers (DNS) used by an ISP can be a bit flakey. I have not used an ISP DNS for years. Try OpenDNS.
https://www.opendns.com/setupguide/Get tough with Comcast (they are terrible). Since the modem was replaced this should be up to them to fix. If the above two suggestions fail insist that a tech be sent out to fix at no charge.