5gHz disables itself

I have an intermittent but very annoying problem. Sometimes, like right now for instance, the 5gHz radio shuts down and won't come back up. I tried a router reboot, I tried explicitly disabling then re-enabling 5gHz, but no luck. I don't see anything interesting from dmesg or in the system log. It shows the half-enabled wifi symbol.

Am I missing something obvious? I'm using an RT-AC66u with 3.0.0.4.270.26, since my three laptops all use Intel 6300 clients and 5gHz - if I read the discussions right, this is not a winning combination for the 354 series firmwares.