Dimension 2350 continually powering up and immediately shutting down.

I'm assuming this is a power supply problem, and I will just have to replace it, but this is such a weird problem I thought I would ask before replacing it.

When you press the power button the fan spins, you can hear the hard drive start to work, and then, the machine powers down, as if it were unplugged.  It stays down for 5 or 6 seconds, before attempting to start itself again.  It continually does this over and over.  Twice yesterday i was able to achieve a normal power up, but today, I can't get it to work.

Before powering down, the ABCD lights look like this:

first A,B,C are off (for a second), and D is yellow

then A and B are yellow and C and D are green

then it powers down.

Any help or information about this is appreciated!