2008 Mac pro shuts down when put to sleep?

Hello all ... I could use some advice.

First, the machine seems to run fine. It doesn't randomly reboot or anything odd.

Running 10.8.2

Macpro 3,1

Single Quad core processor

8 gigs of Apple memory.

Normally, this computer goes to sleep after a certain period of time. When it would go to sleep in the past, I just hit ye old

spacebar and she'd come backup and work great.

Lately (last week or two), I've noticed that, while she does go to sleep, a short time after entering sleep mode, she powers off.

Instead of sleeping.

I leave the computer for 30 minutes (or just put it to sleep), the display shuts off and I see the LED on the front of the ole girl start to flash.

Shortly thereafter, the light goes out.

Using the keyboard won't wake the computer, I have to use the power button. So it's shutting off instead of taking a nap.

Ideas? Things I can test?

Thanks!