I had a similar problem with a DSL line ages ago and what finally fixed it was to upgrade to business-class service, complain to business support, and they sent a tech who eventually found what it was (a tester on the line painted over so as to nearly be invisible). After it was fixed I was able to downgrade back to consumer DSL.

My most recent interaction with comcast business was earlier this year, end of contract came up.

I finally replaced the SB6183 with a Hitron CODA56 to be ready for midsplit upgrade (greatly improved upload speeds which was showing up in advertising on the same road family business is on). The way their sales works now is terrible, they chain you to a specific rep and that rep has to release you if you want to talk to anyone else. It took me something like 4 reps to finally get one that would sell me what I wanted, a no-term contract at list price without the firewall/spyware crap. No promotion requested. Just the 300mbps tier for that site. Nobody anywhere knew when midsplit upgrades would be complete. Thankfully about 2 months later it was done and that location went from 300/20 to 300/300.

Their business tier was better some years ago, now if I have a tech come out they try to charge me every time because I dared to buy my own modems. Thankfully it’s been pretty reliable, better than the power utility (especially since comcast will literally setup honda inverter gens to keep their nodes up in extended outages).