As Apple haven’t yet supplied a fix for this wifi issue in OSX Mountain Lion, I tried another suggestion from a user on Apple Support Forums.
The main gist of this is to clear out all the networking plists from your /Library/Preferences/SystemConfiguration/ directory. Then restart Mac and setup your Network Prefs again.
I tried this and, at first, it did look promising – almost a whole day without any wifi connection issues. But then the next day, the wifi issues returned. Even as I type this blog entry, the wifi connection has dropped out!!!