I've recently bought a Lenovo u400 (http://www.lenovo.com/products/us/laptop/ideapad/u-series/u400/) and installed Ubuntu 12.04 on it.
Everything works fine (I've installed the proprietary ATI drivers for switchable graphics) except that I get a random power down or reboot either while surfing the web or watching videos.
Monitoring the temperature with sensors, the laptop never goes into any critical zone but still may power down or reboot once in a while. Is there any logs that I should check and any particular message I should take note of?
EDIT: This happens to me as well. This also happens without proprietary drivers activated.