Screen freezes after typing first password character

After the boot process finishes, I see the login box from lightdm. The text cursor is blinking in the password box, the mouse cursor responds to the mouse and touchpad, etc.

I type the first character of my password. The text cursor stops blinking (so far, it always freezes *visible*). The mouse cursor moves with the mouse, but nothing responds to clicks. I can switch VTs using CTRL+ALT+F1 and so on, but no other key brings a response, even the power key.

If I switch VTs away and back, the screen initially shows the lightdm screen but redraws to solid aubergine, and is blank other than the mouse cursor. The mouse cursor continues to track mouse movement, and VT switching continues to work, but the X display remains unresponsive in every other way.

For the past few days, this happens about 50% of login attempts. More than a week ago, it never happened.

This is on Precise.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: lightdm 1.1.3-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-17.27-generic 3.2.6
Uname: Linux 3.2.0-17-generic x86_64
ApportVersion: 1.93-0ubuntu2
Architecture: amd64
Date: Mon Feb 27 10:30:36 2012
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Alpha amd64 (20111130.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: Upgraded to precise on 2012-02-27 (0 days ago)