You have what is called the "Random Shutdown Syndrom"
I had the exact same problem with the exact same model of computer. I would be working, and suddenly, it would randomly shut down. It wouldn't matter if it was cold, hot, warm or whatever, I would be working and it would die. I took it into apple 4 times before the fixed it. It was very close to being a lemon.
From what I recall, they had to replace some internal heat sink, then ultimately, the logic board. After they did that, I never saw that problem again.
What I read, what happens is that the heat sink or logic board come in contact with some heat sensor that kills the computer.