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Have you run hardware tests? Put in your OS X disk, reboot holding the D key. Make sure you have an hour or so to do the test.

I've seen a few threads like this on the Apple Support pages. Some sent their notebook in to get parts fixed. Some said it was their battery, others said it was a logic board issue. Some had random shut downs because the notebook itself was overheating.

Is your unit overheating?

Regardless, you should contact Apple.

Here's a good read: http://discussions.apple.com/thread.jspa?messageID=7530860&#7530860
 
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