My Acer laptop has an Atheros wifi chipset. I have to do the following:

$ sudo -s
# echo options ath9k nohwcrypt=1 > /etc/modprobe.d/ath9k.conf

to create the file /etc/modprobe.d/ath9k.conf.

(Sourced from: http://www.htpcbeginner.com/how-to-fix-atheros-ar9285-ar9287-wireless-problems-in-ubuntu-1104/)