My fancy new ASUS RT-AC66U has pretty good firmware; it seems based on the Tomato core, even the QoS. And unfortunately the built in DNS server, the one it advertises in DHCP. It has some bug that occasionally causes it to fail to respond to any requests for awhile, I think blocking on some other connection. I just had a failure where I couldn’t resolve names for about 45 seconds and it went away the moment an active HTTP download of a 60 megabyte file completed. The network was up the whole time (downloading the file), as was the router web interface, but DNS requests failed.
I should probably stop using the router DNS. Or else install Merlin’s patches.