overheating, bad RAM, something faulty on mobo, cpu. It could be any of a number of things. Run some tests, like Memtest 86+, prime 95, ati tool/equivilent tool for nvidia. I would imagine it is heat related, or a faulty part, or both.
My bet is on overheating as well. If your CPU hits a certain temperature it will shut your system down to try and prevent damage. Natravis has coverd just about all applications needed to test this. Memtest 86+ to check for memory errors, Prime95 to test CPU stability and ATI Tool (which works for Nvidia as well) to test your video card's stability.
If your lappy shuts down during one of those tests, run it again to see if you can duplicate the problem. If you can, you have your answer.
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If your lappy shuts down during one of those tests, run it again to see if you can duplicate the problem. If you can, you have your answer.