I have N66U and my 2.4Ghz is not stable since 18E or 19E .
It seems to be the problem is similar to what ls3c6 reported.
In my case 2.4Ghz wireless disconnects , I can see 2.4Gh in the list of networks , but can not connect to it until I reboot the router.
5Ghz works fine and router works good...