@msilenus Good move with the syslog server, that might provide some clues.
I'm not sure what you mean by, still seeing "ISP's DHCP did not function properly". You usually get one of those messages when the router boots up, just before the WAN link is established. So I'm not seeing any other occurrences in the log you posted.
Unless the syslog server reveals something I'm tempted to think it's just the hardware getting old. There have been a few reports of those routers (N66U/AC66U) suffering from "tired" power supplies, causing spontaneous reboots. Replacing the PSU has fixed some of them. What is the quality of your mains electricity supply like?