Marc, has this behavior remained constant across all three modem/router deployments or is it new to this current Arris/Netgear setup?
Is behavior same when multiple devices are cabled into the router (and radio off) as when all are connected wirelessly? I'm assuming everything has been wireless but please confirm.
Please post modem & router model numbers.
Since you just joined today, be aware that BBR partners with ISPs - including Comcast - to offer members direct access to what has proved to be a higher caliber Tech support. Responses are slow but they stick with you until it's fixed and no PR chitchat BS like must be suffered via phone. After years of one guy from Comcast, Steve, being our dreamboat tech he has moved along and Comcast has assigned two new people. Not saying you should go that route but it's an option.
It's over here: »Comcast Direct