Signal quality issue causing intermittent packet loss and consistent network quality fluctuation

I just got off the phone with a Cox technician who told me he was completely unable to schedule a repair for this, so I'm trying here.

I have, since around December, had a daily period mid-afternoon where every other ping or so that I send out times out waiting for reply. This causes my internet connection to slow to a crawl and renders it borderline useless. Seeing this, I looked at my modem statistics and noticed that my signal levels and error rates are abysmal. I bought a replacement modem and installed it, and the signal is still as bad, so I know it's not an issue with the modem in question. I called Cox support and the tech said that he saw nothing in diagnostics, and therefore could not schedule someone to look at the line out by me. This is impacting my ability to work from home and I need this resolved soon or I will have no other way forward but to find a new ISP.

My current modem statistics as of this post are:

Model: SB6183

Downstream channels, 16 bonded:

SNR ranges from 38.7 to 39.7dB

Power ranges from -1.3 to 0.9dBmV

Corrected codewords from 83 to 358, uptime of 4 days.

Uncorrectable codewords ranges from 0 to 103, modem uptime of 4 days.

Upstream channels, 1 bonded (on a 16x4 modem, which is terrible):

Power: 55.5dBmV (equally as terrible)

I have had some T3 timeouts occur as well, but those were cleared out of the event log so I do not have evidence of their occurrence.

As you can see, aside from the ridiculous amount of corrected/uncorrectable codewords I'm receiving downstream, my signal strength is decently acceptable (SNR is a little low, I suppose) but my upstream is absolutely abysmal, to the point that I can't even feel confident I'll maintain an upstream connection at all if it gets any worse. Please let me know how I'm supposed to get a line tech to check the lines out here, since according to the phone tech I was on with, there's "no issue" to check.

As an aside, my connection is also testing at 100/15, despite me being on the 150mbps plan. While understandable that I'm not guaranteed a full 150, to be only at 2/3rds of the speed I'm paying for in a metropolitan area also has me concerned about the line quality.