TWC's been out on multiple occasions to check the line, so I assume signal strength is good. As for the modem, they've given me the same one a couple times in a row - not sure if I could request a different one. I tried testing with the cable disconnected, and it did seem to break the boot loop. In detail:
1) both modem & router are on
2) power-cycle router
3) router comes up & then goes into reboot loop
4) pull cable from modem
5) router stops rebooting
6) power-cycle router
7) router comes up, and does not reboot
reattach cable
9) router continues to function normally
This sort of agrees with my initial wholly-intuitive, non-data-based diagnosis: That the modem was generating / passing some sort of heavy traffic that caused the router to choke on boot. Unfortunately, there's no way to connect the router directly to the internet, to see if it's just heavy P2P connection attempts or whatever, or actually something specific to the modem.
I can say that this was not a problem pre-Arris. The router worked fine with the previous modem on the same cable. Also, this Arris modem has been replaced once or twice, with the same results.
Anyway, I'm moving to an Optimum Online area later this year, so I guess that will be a whole 'nother experiment.