Perhaps a dupe here. The tricks below worked for me a day but now they are again not working, irritating! So this case is not resolved at least with Thinkpad X220.
I. Generic partial solution
The tip by this article
does not solve it but makes it a bit better:
$ sudo /etc/init.d/networking restart
The fix is still missing, investigating.
II. Some Intel models
Apparently a fix by Chili555
$ sudo modprobe -r iwlwifi
$ sudo modprobe iwlwifi 11n_disable=1
so send feedback to Intel to fix this stupid recurring bug, this is
old! The latter apparently works for Intel Ultimate-N 6300. I tested this on X220 Lenovo Thinkpad -laptop and it worked, hopefully a lasting solution -- this makes very bad reputation to Intel as non-professional provider!
Perhaps related, a ton of material in google but hard to find the relevant
Some summary of things, looks similar as my found solutions or hacks,
[BUG] The recurring bug already filled by Redhat here.
[RESOLVED] Dell 1390 wireless (BCM4311) Ubuntu 12.04 no wireless icon in unity
[Reported? Bug?] Cannot connect to WLAN with increased frequency
[Resolved, related?] https://answers.launchpad.net/ubuntu/+source/gnome-nettool/+question/199216