Q: Yosemite wifi problems

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.

There is an awful lot of misguided misinformation on this subject that is causing a lot people to waste time with ineffective solutions. I am currently in a hotel with numerous neighbors generating wifi signals in the 2.4Ghz band. This is the problem. For whatever reason, Yosemite will interrupt your WiFi session when your receiver detects a strong enough interfering signal. Take a look under the WiFi icon to see what other signals are within range. I would bet that you will see that you are picking up wifi signals other than your own router. Other than lining your interior walls with aluminum foil, one solution is to upgrade to 802.11n or ac that operates in the 5 Ghz band, try moving things around or wait for a fix from Apple. No doubt they are aware and are working on a solution but no solution can  be released without thorough testing for fear of creating even worse problems. There are all kinds of things that can create interference in the 2.4 Ghz band but the 5 Ghz band is relatively sterile. In my case, I can identify the individual wifi router that was causing my problem. When it was operating, constant interruptions, when the owner disconnected it the problem disappeared. Take a look at this Wired article for a nice explanation. http://www.wired.com/2010/09/wireless-explainer/all/

c.doppler

I am having the same issue.  Since update to latest on 12/7/14 wi-fi is slow and sometimes drops altogether.  I have noticed on start up I have to turn wi-fi off and and then back on to even access the internet.