What could be wrong with my Mac Mini?

I own a 1.25 GHz Mac Mini bought from Apple refurb 4 months ago.
What happens is while using it the screen just goes blank and the light brightens/dims on the mini. if I press the power button it will come back for awhile. According to the Apple docs, the pulsating light indicates it is going into energy saving mode. I have this feature turned off in system preferences.

I sent it off to an authorized Apple repair place. After 3 weeks they finally replaced the mainboard and I just got it back today. Same crap. I have put the original RAM back in, no difference. I put the old hard drive back in. Same crap. I know they changed the mainboard because my serial number is wiped and it was at 1.33 GHZ now its at 1.25.

So essentialy, everything in this is new. WTF? Is this thing haunted or what else is left to make this thing do this???

 

No I have not. Do you really think this is a possibility? I would have to go borrow one to try.

Anyone else ever have this sort of thing happen?