This has happened to me a few times doing different things. Most recently, it has been things such as installing an operating system (Windows 2000 and Linux) compiling software, or ripping a dvd, which are all cpu intensive activities. When this happens, the computer goes into sleep mode or completely powers down. I'm not sure exactly what to do... I've encountered this problem quite often, and have only found two workarounds, one, is to make sure that the bios is set to "maximize battery life" when there is no ac adapter, and that will throttle the cpu back enough that it doesn't overheat and turn off, or just press the power button whenever it happens. Anybody could tell me why it does this and how I could fix it?