I just read this: The cause is heat. The SMC fan is not cooling down the system which is then shutting down because of the heat.
There is a temperature sensor in the drive to supposedly prevent this. However, if you've changed your drive like I did, and replaced it with a drive that did not have a temperature sensor, thent he fan controll gets all confused, doesn't turn on properly, everything heats up and then shuts down.
This also happens on factory installed drives when the temperature sensor goes bust. A lot of Apple's early drives for iMacs and MacBooks were really cheap and the sensor failed. I had to replace my factory isntalled drive because it failed. (Thank you time machine).
The solution is a simple FREE software download called SMCfancontroller that turns your fan on at specific RPMs, and turns it off when you aren't overheating. I installed the software and haven't had a "go to black" moment since. The software can be downloaded here: http://www.macupdate.com/app/mac/23049/smcfancontrol