790 problem

Hi,

My 790 Mini Tower has been running flawlessly for ages ... up until a few days ago.

Now, most of the time, when I press the on/off button nothing happens.

When I hit the test power supply button in the back it lights up green and I can hear the fans working.

Every once in a great while it will turn on... for literally about three seconds. 

The diagnostic lights go from 124 to 34 and last to 3. Then it dies.

The blue hard drive activity light functions normally in this very brief timespan.

What do you think? Power button wiring? Motherboard? What else could it be?

I don't want to start buying replacement parts until I'm at least pretty sure what the problem is.

Any help with this will be appreciated.

Thank you.