the reason id suggested connecting your modem at the outside drop would also cover any and all inside wiring issues. and that could be anything. frayed end. a nail through it. moisture.

i think my inside wiring was done no earlier than 1991 , but maybe redone once since then and it looked pretty good but i found this on the back of a wallplate , just yesterday:

https://ibb.co/5XjkJ57J

- expires in 6 months

the easiest thing to do is check it at the box and then if nothing else thats ammo for dealing with customer service “look, i connected at the drop and have the same issue its NOT my inside wiring.”

everything from that point back is their problem and they owe you bill credits until its fixed, so get the proof and go back with it.

in my case my modem worked perfectly at the drop :D so i unfortunately had some digging to do

its not practical to suggest someone on the internet go ripping open all their wall plates and checking every inch of inside wire or maybe even running a new one. unless it passes the drop test, and then yeah, thats what needs to be done.

but plugging into the drop will 100% prove whose problem this is. youre california so thats also good ammo for a PUC complaint, that you did that and proved its not your inside wiring and now theyre refusing to deal with it. maybe that will get it to the right person on comcasts end faster when they review it.