iMac goes to sleep in Mountain Lion and won't wake up

24" 2.8 GHz Intel Core 2 Duo iMac, 4GB RAM, 1TB HD

After doing a CLEAN install of Mountain Lion on a partition, the system goes to sleep mode (monitor shuts off, motor runs, fan runs hard for a bit, but then all are silent - however it's still powered on because the keyboard and mouse are still on) after 20 minutes EVEN IF I'M IN THE MIDDLE OF USING IT and then does not wake up no matter what I do. I have to power down and restart it.

This actually happened DURING installation, TWICE. The first time I powered down and restarted it, and it restarted the install process. The second time was while migrating files from another disk (Lion partition on the same HD). When I restarted the computer that time, it took me through the install process again, but when it got to the migration part, it said I only had 18GB remaining, meaning it had transferred all of the files while it was asleep. I had to make a new user, load the system, then switch users to go to the existing user migrated over from Lion.

Things I've checked/done:

Updated the software

Energy Save is set to NEVER, does not put hard disks to sleep, wakes for network access, no allow power button to put computer to sleep.

Reset SMC

Changed the hibernatemode using the 'man pmset'

Any suggestions?