CM1000v2 Xfinity losing connection intermittently, dropping to 0.1mbps.

Hey all, I've been having some issues with Xfinity internet recently. I'm paying for the 600mbps internet, however on a good day I'd be lucky to get 100. Every hour or so (sometimes even multiple times an hour) the internet speeds will drop to 1mbps and then slowly deteriorate down to 0.1mbps before stopping entirely. I suspect it has something to do with the wiring outside my home, as the wires have been here since I moved in 20 years ago. It seems like the downstream power is quite low, and the high number of uncorrected codewords concern me but I'm not too knowledgable in this area. I have a tech coming in to check it out for the third time, but I wanted to have someone on here look over the logs and give me some more info in case the tech tries to blame it on my hardware and leaving without fixing anything again. Would appreciate any help or info on what could be the cause. Modem is brand new, tried to swap it out with a different one to make sure the modem wasn't faulty, but the problem still persists. Thanks in advance!