reply to hardware bum
Re: DHCP troubles "So long as the modem is plugged into the WAN port of the router, clients should never get an IP from the modem."
That's what I thought, too.
"You might look at the RT-N66U's setup to see if you see anything resembling a DHCP relay setting checked."
I don't see anything like this in the RT-N66U configuration. (Not saying it's not there)
"Are your modem, router and these other devices all plugged into a switch?"
There is a downstream switch.
cable >> modem >> NT-R66U (wan port) >> fan out in several branches
Wiring has been stable for 6 months or more. This problem started about a month ago.