Sudden connectivity problems with 5Ghz/40mhz

I've been running a 5Ghz AP using 40mhz for a while now with no issues. All of a sudden, though, I'm seeing some strange behavior. Certain devices (not all) either get really high latency or can't connect to the internet at all. After a ton of fiddling I finally changed it to 20mhz and everything worked fine.

I'm running a RT-N66U with Shibby v121 AIO 64k. Any ideas what might be going on here?