Q: iMac continuous shutdown?

Specs: 24-inch iMac 3.06GHz Intel Core 2 Duo, running OS X 10.7.3.

I have been having recurring shutdowns of my Mac. Resetted PRAM, pulled/plugged in power cord, ran permissions/repairs, cleaned caches, and I have 2 Apple memory modules and two third parties, for a total of 12 gygabites.

CPU runs at an even 1800+ RPMs, temp 70/80 degrees (with warning). When it shuts down, it doesn't automatically restart, need to do it manually. Small progress bar appears on the bottom (never seen before), and it dumps me into the log on window, which I don't have selected as an option.

That's all I can document. Any clues?. Thanks in advance

P.S. Only suggestion to the problem I found on the web is to up RAM. Valid point?, if so, how much?.