There's currently an issue with Comcast and IPv6 where it fills up your routing tables. I suspect this might be leading to resource exhaustions, followed by a router reboot.
Comcast blames Asus, and Asus blames Comcast. People in the middle of them blame the old kernel used by Asuswrt. So at this point, there is no real solution if that's the same thing affecting you.
For this prolblem, the following script in /jffs/scripts/init-start works for me (I'm also on Comcast using the RT-N66U):
#!/bin/sh
echo 512 > /proc/sys/net/ipv6/neigh/default/gc_thresh1
echo 1024 > /proc/sys/net/ipv6/neigh/default/gc_thresh2
echo 2048 > /proc/sys/net/ipv6/neigh/default/gc_thresh3
touch /tmp/init-start.ran
I was getting "neighbor table overflows" regularly until I started using the above script, and now I don't. So if this is the problem causing your reboots, you might try the above script and see if it helps. You can increase the values for the threshholds that are used up to a point, this one is experimental.
I'm not seeing this problem on the Netgear R7000 wihen I have IPv6 enabled.