I fixed the problem in my Mac Pro.
These were the steps I followed:
- reinstalled Yosemite twice. This didn't help because Apple force all the updates to be included in any new installation, including the one that breaks the WiFi.
- tried to install Windows 7 from a DVD. I couldn't really pass from the initial screen because the mouse and keyboard didn't work, but I'm mentioning this because I can't tell if this had any effect on the network card.
- installed Windows 8 with Boot Camp
- formatted everything again and reinstalled Yosemite 10.10.3, the connection is perfect for 4 days in a row
I'm still having a hard time to believe Windows fixed a Mac Pro, but I'm not the first one who tried and was successful.
Hope this helps someone