Hi all,
i've just had the same problem, what i did to fix it was to jump onto the management page of my WIFI router and change the channel setting the Router users and fixed it to a channel below 10 (it's an Asus dual band router and it phrases this as a control channel). If you then set the extension channel to use a channel below the control channel this should fix the problem too. if the extension channel goes into channels 12+13 then you'll probably see the you're PI working either slowly or not at all!
This problem not only stopped my PI from working but also stopped a nannycam working as well!
One security problem that you may encounter by fixing the channel if there are wifi routers nearby then they could interfere if both channels are set the same. there are other issues but that would make this post way to long.
hope this helps
Jayvr