Q: imac keeps dropping wifi

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.

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 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 only

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. Turn Wi-Fi off and then back on. You'll be prompted for the password when you reconnect. Save it in the keychain.

Step 10

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

I bought a 27inch iMac with 5K retina display just before christmas.  all was working fine until the day i tried to download an iOS update for my phone and the wifi dropped.  the only way to get it back is to restart.  it will then get a great signal for a minute or so, then drop again.  according to network preferences, it is connected fine with no issues.

it's happened multiple times every day since, to the extent that i am now using my old MacBook for internet applications.

i did live chat with Apple for about 2 hours over this, using the macbook again as the connection dropped, and they couldn't figure it out.  we ended up running some diagnostics and they sent me a link to upload the files.  the link didn't work so i couldn't upload the files.  mailed them back and had no reply.

My home wifi works with no issue at all on the macbook, iPad, iPhone and even the cranky old windows laptop.

Step 2 in Linc Davis' post did it for me: I had the same problem as you yellow6 today, while my new iMac 5K retina had been working perfectly for the four days that I've been using it. But today I had connected my iPad to it. I disconnected the iPad (and another usb stick that I had, but that did not create any problems yesterday), and the connection issue has gone. Will experiment more, and look for a solution (surely I want to be able to connect the iPad to the iMac!), but for the moment disconnecting the iPad seems to have done the trick!