If your MacBook (13-inch) shuts down randomly during use, a downloadable firmware update is now available that resolves this issue.
The MacBook SMC Firmware Update v1.1 improves the MacBook's internal monitoring system and addresses issues with unexpected shutdowns. This update is recommended for all MacBook systems, including those that received warranty repair.
Note: If your MacBook is running Mac OS X 10.4.6, you won't be able to see the SMC Update until you update to Mac OS X 10.4.8. Once you have updated your system software, the SMC Update will appear as an option in Software Updates.
MacBooks running Mac OS X 10.4.7 will be able to download and install the SMC Update, so it is not essential that you update your system software, although Apple always recommends updating to the latest version of the system software.
If your MacBook is in a state where it will not remain on long enough to install the update, wait for the issue to reproduce itself. Once the MacBook has turned itself off, turn the computer on by holding the power button down. Keep the button down until you hear a single long tone. Once you hear the tone you can release the button, install the recommended updates, and restart normally.
If you continue to experience trouble with your MacBook after applying both updates please contactAppleCare for further assistance.