sounds like you're suffering from the random shutdown issue.
When it shuts down, does it just go black? no warning?
What surface are you using it on? (ie does it feel hot?)
Whats the CPU load like when it shuts down? (check via Applications -> Utilities -> Activity Monitor, and click on the CPU tab near the bottom)
If the last two steps indicate its getting too hot, it may be Intels protection firmware shutting the system down before the CPUs melt.
If it just goes black with no apparent reason (not hot, not a lot of load, etc.), take it back, say its shutting down randomly, and they should replace the logic board for you.
I had the same problems, and i had the board replaced, and i haven't had a problem since.