... And what a coincidince would it be that a neighbor decided today is the day to install a wireless router so strong that it blasts away the AE signal (the AE is 4 meters away).
Neighbors are the worst sort of household pest. You can't exterminate them.
This is the problem... your neighbor's 2.4 GHz network. You did everything reasonable to rule out any other possibility. You could ask him to reduce his base station's power so that it does not interfere with yours, but other than using your 5 GHz network that is all you can do.
Excess transmitter power is simply a waste. It's like yelling in a quiet room.
Use what you need and nothing more. Of course no one does this; it's like subwoofers in cars. Good luck telling them to turn it down.
I've used AE's since the originial base station came out. I love them.
but one day I came home and found out that the WiFi doesn't work anymore. the same thing that happened to the original poster - happened to me, someone installed a powerful router and basically jammed the AE. I couldn't find a clean channel. I do have some devices that can only operate on 2.4Ghz Wifi so I have to stick with this frequency.
after 2 days of trying everything: using iStumbler, trying every channel available, moving the AE around, resetting to factory settings and even using a second AE as an extender... everything failed. the only solution was to switch to 5Ghz (which worked great) and lose the 2.4Ghz devices. I've decided to try one last thing and simply buy a new router (if it wouldn't have worked - I'd return it). I bought this Netgear Nighthawk and the minute I set it up, I knew I've done the right thing. the Wifi started FLYING. got excellent connection. it's a dual band so now I have both 2.4ghz and 5ghz working ! oh - and it supports Time Machine. what more would an apple user want?
I am sorry, apple but looks like one of the other guys got it better this time. at least for me.