Your suggestion of resetting the SMC & PRAM on my iMAC did not eliminate the intermittent power shut down events. It happened again within a couple of hours of my performing the steps you recommended.
OS X Version 10.7.5
Tried this and unit still shuts down a couple of times per day. Have not noticed any trends or certain running programs when it happens. Requires complete power reboot to get started. May be time for new MAC.