Having installed Ubuntu 12.04 on that Lenovo box, which has an nVidia graphics chip, we find there’s an error somewhere inside the current 295.40 (and perhaps previous versions) of the proprietary nVidia driver that causes random video lockups which generally require rebooting that sucker. Of course, the default Unity desktop requires that driver for 3D operations like compositing, because the Free Software drivers don’t / can’t do 3D in hardware.
How is it that a (nominally) Open Source / Free Software OS requires proprietary drivers just to present the UI? Oh, right, 3D is glitzy and that’s what matters most in these degenerate days.
The least-likely-to-fail solution seems to be disabling the nVidia driver, which enables the Nouveau driver, which does 2D just fine, which lets Unity stumble along. Reverting to 295.33 seems to work for some folks, but I have other things to do…