Asus RT-AC68U reboots every 2 days since version 374.40.
Version 374.42_2 did not fixed the issue.
I am checking the logs but no good information is there.
I am using Optware with transmission, minidlna, samba, 500MB SWAP, no OC, temperatures are good (2.4 GHz: 55°C - 5 GHz: 56°C - CPU: 85°C).
It gets really frustrating!
I get well over a week of uptime on my own RT-AC68U (before I had to reboot it with a new test build). Try not using Transmission and minidlna for a few days to track down the cause.