Finally got some time to diagnose a network issue that has been plaguing me since Wednesday.
Symptom: sporadic reset of modem, seemingly associated with connecting/disconnecting devices from network, reset occurring every few minutes.
Router showing a large buildup of dns requests from devices on the network before resets.
After isolating different segments, observing traffic, shutting down various units, ...
Turns out it wasn't any device on the network, none of the routers or switches. It wasn't the modem (though that's aging enough to warrant replacement).
Was an overheating power supply. Sized inappropriately for the modem. Swapped with a newer power supply of same inadequacy, just newer as a temporary stopgap to get network stable until i replace the modem entirely
