To give the tech the benefit of the doubt, it's possible that he just isn't certain he can trust other speed tests. They may point to issues beyond his control.
In my case, the TWC guys replaced every piece of coax and splitter from my cable modem and TVs to the pole.
Turned out the issue was the changed out the cabling on the big avenue near my house and used the wrong grade of cable.
I called the TWC state lobbyist and told him I represented 150 homeowners who were filing a complaint with the regulator and the city council just head of their franchise agreement renewal. Trucks were replacing those lines the next morning.
> In my case, the TWC guys replaced every piece of coax and splitter from my cable modem and TVs to the pole.
I am surprised by the coax inside the house. Usually they'll help if they can, but your house wiring is your problem. (Unless their solution is stapling a coax cable to your baseboard.)
The techs were observing erratic behavior, but there wasn't any evidence of bad cabling -- I had actually replaced it about 3 years before. Also, my wiring is low-complexity... I terminate everything in the basement, and have 1 TV. The TV cable ends about 6 feet from the cable box.
Even in other circumstances, the techs are pretty good.
I had an issue with Comcast last year where I would have large latency spikes for ~2 hours every night, and speedtest.net always remained at normal latency: https://i.fluffy.cc/kNCxLMbkF2wx7NHFJxbzcgX35Bsb5nvl.png
I couldn't find any non-speedtest sites where my latency was less than 100ms, but Comcast's own speed test, several other public speed tests, speedtest.net, and even ookla.com (the company behind speedtest.net) were perfect (the ~20ms you see on the graph).
On a separate note, what tool did you use to create that graph?
[1] https://github.com/cfallin/pingd (in Rust, requires liboping)
What you describe is probably congestion in the network somewhere that isn't the last mile. Speed tests aren't a good indicator for any issues occurring anywhere but the last mile, as they won't take the same path with fairly high probability.
Really, my point was that congestion is often localized in a network. The fact that it isn't present in a path doesn't mean it's an ISP gaming speed tests - though perhaps they'd be more keen on peering directly and avoiding congested transit if it makes them look better, and I would absolutely believe that would happen.