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.
Step 2
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.
Be sure to test on more than one Wi-Fi network, if at all possible. A desktop computer can be tested with a mobile hotspot, such as the hotspot created by a cell phone. If the problem only exists on one network, then the access point of that network should be the focus of your attention, not the computer.
Step 3
If your model supports AirDrop (most do), open the AirDrop window and select
Allow me to be discovered by: No One
Step 4
Disconnect all USB 3 devices, and also all external displays if the Mac has one built in. If you don't know which devices are USB 3, disconnect all USB devices except keyboard and mouse.
Step 5
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 6
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 7
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 8
In the Wi-Fi settings, select
Advanced... ▹ TCP/IP ▹ Configure IPv6: Link-local only
Click OK and then Apply.
Step 9
Reset the System Management Controller.
Step 10
Reset the PRAM.
Step 11
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. Turn Wi-Fi off and then back on. You'll be prompted for the password when you reconnect. Save it in the keychain.
Step 12
Disconnect all wired peripherals except those needed to start up, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in.
Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output. The next normal startup may also be somewhat slow.
The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. If the problem is (temporarily) resolved, stop here and post your results.
Step 13
Reinstall the OS. The linked support article refers to OS X 10.11 ("El Capitan"), but the procedure is the same for OS X 10.7 ("Lion") and later.
Step 14
Make a "Genius" appointment at an Apple Store, or go to another authorized service center.
