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There are several reasons for this problem, may be even hardware related like a WiFi chip issue of Macbook Air. We can do some troubleshooting steps and see how it goes.
First reset the System Management Controller of your Mac. Follow the steps below;
Shut down the computer.
Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.
Then reset the PRAM and NVRAM of your Mac. Follow the steps below;
Shut down your Mac
Restart and immediately hold down Command-Option-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup. This will reset the PRAM.
Then reset the network configuration files. In Finder, click on Go menu and select Go To Folder. Then enter the following command and press return
This will open SystemConfiguration folder. Then trash the following files
Restart your Mac. Check if it is dropping conection now. Please let me know if that didn't work, so we can continue troubleshooting.