After upgrade the computer from 13.10 to Ubuntu 14.04, the connection has been really unstable. The connection frequently drops, fails to connect, or is very slow.
You should deactivate the N-mode of the driver.
Open a Terminal “CTRL+ALT+T” and type this command line:
# sudo apt-update
# echo "options iwlwifi 11n_disable=1" | sudo tee /etc/modprobe.d/iwlwifi.conf
# sudo modprobe -rfv iwlwifi
# sudo modprobe -v iwlwifi
Note:
Additionally you can deactivate the power management of the card:
# sudo iwconfig wlan0 power off
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