02-12-2011, 10:46 PM
When a Mac desktop is spontaneously shutting down without an error message, the first thing that I suspect is a bad power supply.
When a Mac laptop is shutting down without an error message, the first thing that I suspect is a bad battery and/or energy-saving circuit.
You may want to download this program and see what it has to say about the health of your battery/energy-saving circuit:
coconutBattery (free, donations accepted)
coconutBattery 2.7 - by coconut-flavour.com
You may want to try re-calibrating your energy-saving circuitry:
Apple Portables: Calibrating your computer's battery for best performance
Randy B. Singer
Co-author of The Macintosh Bible (4th, 5th, and 6th editions)
Macintosh OS X Routine Maintenance
OS X Maintenance And Troubleshooting