I have the same problem. And in my case it happens when i transfer "large" files over the WiFi. When i send an MP3 filer over Skype i loose connection. When i transfer updates between my iPhone 5 / iPad 3 and iMac with Mountain Lion i loose connection. When i have a video conference i loose connection. You get the idea.
There is usually like 30 people using their own router on the 2.4 GHz band near by. None on the 5 GHz band which i use. But when my WiFi freezes on ML i can only see my own router. And i am also connected to it. But all my neighbors WiFi routers are now invisble to me. My network stalled.
What i do is that i disconnect my WiFi and then reconnect it again. Thank fixes it. But all my transfers and what not has been dropped. And i have to start all over - and the WiFi will stall again.
While my WiFi is frozen on the iMac with ML i have no problem using the same router on the same 5 GHz band with my iPhone 5 or iPad 3.
Downgrading the AirPort Extreme to 7.6.1 from 7.6.3 helped a bit. Now i only loose connection a few times each day.
It is a software error in ML i am sure of. A driver maybe. But i am suspecting that it is either a speed-sync error or an energy saving error. Like the network card turns of in the middle of a file transfer to save enrgy - maybe.
When i stream movies from my iMac to my Apple TV or 3 there is no problem at all. It is so odd.
