i once had a similar problem, it turned out in the end that the fan had a blade broken and caused the computer to over heat after a while, but our first huess was that my friend tried to screw with some of the RAM and wasn't properly grounded so the comp shut off because it did not have enough RAM to run on. this turned out to not have caused the problem, but he did screw up the RAM, still havn't gotten around to replacing it. anyway, you can try those two, see if you get anything.
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