A computer shutdown, whether you see a BSOD, or not, is most often related to hardware, and most often to overheated hardware.
Take a look at this ARTICLE, for some tips, and tools.
After all possible hardware causes have been exhausted (but not until you have tested them all), then you will probably be looking at a driver issue with the OS.
Thanks Hunt, I'll look into that. The overheating seems like the most likely culprit, but what I find strange is the inconsistency and how one after effects element seems to completely bog down the rendering process (and result in a much 'hotter' render as well). I do have a few things running off a mapped eSata drive -- is there any change that consolidating everything into on drive/folder could help?
The eSATA should not be an issue here. Especially with a laptop, you should look into as much cooling, as you can. I have a Zalman cooler pad, run at full-tilt, plus SpeedFan, kicking things up a notch. With some Exports on hot, AZ Summer days, I have even placed an ice pack on my externals, to keep 'em cool. Insulate well, as you do not want any moisture onto the drive.
Good luck, and hope that is all that it is.