Q: Frustratingly, since I upgraded to Yosemite 10.10, I too am having the worst problems with WiFi dropping out. I've never had this problem before on my iMac 27-inch mid 2011 model. Turning WiFi off and then back on again sometimes works. Help please.

Please test after taking each of the following steps that you haven't already tried. Stop when the problem is resolved. Back up all data before making any changes.

Step 1

Take the applicable steps in this support article. The Wireless Diagnostics program generates a large file of information about your system, which would be used by Apple Engineering in case of a support incident. Don't post the contents here.

Step 2

Disconnect all USB 3 devices. If you don't know which are USB 3, disconnect all USB devices except keyboard and mouse.

Step 3

If you're not using a wireless keyboard or trackpad, disable Bluetooth by selecting Turn Bluetooth Off from the menu with the Bluetooth icon. If you don't have that menu, open the Bluetooth preference pane in System Preferences and check the box marked Show Bluetooth in menu bar. Test. If you find that Wi-Fi works better with Bluetooth disabled, you should use the 5 GHz Wi-Fi band. Your router may not support it; in that case, you need a new router.

Step 4

Open the Energy Saver pane in System Preferences and unlock the settings, if necessary. Select the Power Adapter  tab, if there is one. Uncheck the box marked

          Wake for Wi-Fi network access

if it's checked.

Step 5

Open the Network pane in System Preferences and make a note of your settings in the Wi-Fi service. It may be helpful to take screenshots of the various tabs in the preference pane. If the preference pane is locked, unlock it by clicking the padlock icon and entering your administrator password. Delete Wi-Fi from the service list on the left by selecting it and clicking the minus-sign button at the bottom. Then recreate the service by clicking the plus-sign button and following the prompts.

Step 6

In the Wi-Fi settings, select

          Advanced... ▹ TCP/IP ▹ Configure IPv6: Link-local

Click OK and then Apply.

Step 7

Reset the System Management Controller.

Step 8

Reset the PRAM.

Step 9

Launch the Keychain Access application. Search for and delete all AirPort network password items that refer to the network. Make a note of the password first.

Step 10

Make a "Genius" appointment at an Apple Store, or go to another authorized service center.

If following Linc's directions doesn't help your situation you might look at this web article: Fix Wi-Fi Problems in OS X Yosemite.

There are definite bugs in Yosemite that need to be addressed by Apple. Multiple people are experiencing Wifi issues with fresh installs of Yosemite.

I was experiencing consistent Wifi issues after a fresh install of Yosemite.  I validated that it wasn't my computer or router by doing a fresh re-install of Mavericks.  Wifi works flawlessly under Mavericks but always fails under Yosemite. No other software was installed as part of this test.

Here is my configuration:

Model Name: MacBook Pro

  Model Identifier: MacBookPro8,1

  Processor Name: Intel Core i5

  Processor Speed: 2.4 GHz

  Number of Processors: 1

  Total Number of Cores: 2

  L2 Cache (per Core): 256 KB

  L3 Cache: 3 MB

  Memory: 10 GB

  Boot ROM Version: MBP81.0047.B27

  SMC Version (system): 1.68f99

  Serial Number (system): C2QGV006DVHJ

  Hardware UUID: XXXXX

  Sudden Motion Sensor:

  State: Enabled