My Airport Extreme has this problem since I installed Yosemite and I have tried virtually every fix in this thread and from other forums. Ditto the bug fixes. So buying more Apple products is not a fix and not recommended by me.
Genius Bar techs are helpless because they can't duplicate the problem.
This is not the first time Apple has had WiFi problems on upgrades but it is surely the worst.
At the time of this writing, now months after the release, I find it a bit incredible I and other users are still struggling with it, but I can say this to Apple if they are listening:
1. Long time users are very dissatisfied and losing faith in your ability.
2. Please get some fresh blood in your SW team, you need people with a better understanding of WiFi and Bluetooth because this is getting old.
3. I will never buy or recommend an Apple wireless hub again because they are overpriced, laggard technology and no guarantee of compatibility.
4. I'd like to say I will never buy another Apple PC again but my investment in multiple PCs and software is so great already I have no choice but to continue, but do so as an unhappy customer.
5. I curse your existence every time I have to debug and reset what should be a reliable and automatic service, which wastes my time not yours. This makes me also a resentful customer, which is just about the worst kind to have.
Anyone else reading that has a new proposed fix, thanks for positing it, I will try anything. We are on our own here. Apple is clearly no help.