Ok, it's my shitty cablemodem ISP. Works fine when I use firefox via Socks5 proxy. Probably fine from a VPN as well, but I don't have one to try.

I then launched, on the problematic cablemodem ISP connection: $ chromium --enable-ssl-debugging

Then looked at the chrome://net-export/ https://netlog-viewer.appspot.com/#events

And put a few of the relevant events into:

https://pastebin.com/ZuH4vxQW

Hell if I know how to decipher / troubleshoot that, but thought I would share.

I'll remember to try a different network the next time something like this occurs.