wifiguru - also, I found this in my notes, i.e. i ran the Apple Hardware Test when I first detected this problem upon installation of Yosemite:
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11.04.2014 after returning from Romania, wifi has become intermittent.
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Seems like a hardware error in which two problems occur:
1) laptop doesn't see ANY of the 20+ networks in the building that it usually does.
2) laptop does see those 20 networks, but can't connect to mine.
GOAL: run AHT (apple hardware test): http://support.apple.com/en-us/HT1509
1) Shut down your computer. Then, press the power button to turn it back on.
2) Press and hold the D key before the gray startup screen appears.
It takes a minute or so for AHT to start up and inspect your hardware
To exit AHT, click Restart or Shut Down at the bottom of the window.
RESULT: no problems found.
then found a built-in wifi diag:
http://www.computerworld.com/article/2476307/mac-os-x/os-x-mavericks--using-appl e-s-hidden-wi-fi-diagnostics-tool.html