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Apple has changed a lot under the hood regarding WiFi in 10.10. The current public beta seed notes that testers should focus on WiFi issues in the 10.10.1 beta seed, so I guess there are more people out there having issue with WiFi under 10.10. Yours sounds oddly familiar, though, like a textbook Bluetooth+WiFi interference issue.
Did you start using the Wii Remote under 10.10 - or did you also use it under 10.9?
The band (2.4 or 5 GHz) used by your WiFi is something you set in your router. If your router supports 5 GHz networks, you should be able to change the band in its settings (often a web interface).
Before doing so, however, please verify your Mac supports 5 GHz networks - or that you have another way of connecting to your router, so you have a chance to reverse that setting if not all of your devices support 5 GHz networks.
Some routers also support 2.4 GHz and 5 GHz WiFis at the same time. If yours supports that, I recommend giving the 5 GHz network a different name, so you (and your devices) can tell the two apart.
