It is not guaranteed by Comcast, so I would not like to rely on it. A guaranteed static IP address is an additional monthly charge, I think a substantial one.
--- rant on ---
If this issue is caused by the failure of the modem to reset the LAN link after recovering from a failure (which seems plausible), it's hard to know what to call it. Is it a flaw in the modem (for not resetting) or in the router (for not adapting somehow). Which manufacturer to hammer on, trying to get a solution?
And if the only solution is to throw money at the problem? Replace the modem? Replace the router? Replace both? Try a Netgear router to go with the Netgear modem? Try a commercial router with better tech support? We have voice service as part of our Comcast package, and at present, they offer only TWO modems compatible with that . . . both by Netgear.
--- rant off ---
Thanks to all who have tried to help. I'm hopeful that changing the DHCP query frequency to "Aggressive mode" did indeed help, but there is no way to know for sure. Maybe disconnect the cable for a moment and see what happens?
If you are going to disconnect something, disconnect the coax so you can observe what the modem and router do. If the modem dose not drop the ethernet interface, your modem is at fault. If the modem dose drop the ethernet interface and the router dose not reconnect on coax connection then the router is at fault.
There is another possibility, somebody is stepping on your WAN IP and confusing the ISP's infrastructure's ARP tables. This is a known security issue with DHCP. Some ISPs defend agents this better than others.
Good luck,
Morris