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.
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I took the battery out and have not seemed to have a problem with it since.
crwdns2934271:0crwdnd2934271:0 Trent crwdne2934271:0
mayer - I downloaded the application and the it says the battery is at 76% of original capacity and has about 285 cycles on it. It seems to just happen as long as the battery is in the computer, if it is running solely on battery it does it, if the battery is in and it is hooked up to the charger it does it, however if the battery is out and it is hooked to the charger it seems to run fine. I restarted it with the battery and it started fine, then I took the battery out and it restarted just fine too.
crwdns2934271:0crwdnd2934271:0 Trent crwdne2934271:0
I am having this issue now, on a Mac I have literally JUST updated to Snow Leopard. It didn't do it before? As in an hour ago??
crwdns2934271:0crwdnd2934271:0 Sander Koop crwdne2934271:0