Dimension 2400 - AB lights yellow (amber)

Ok so I have a 2400 that will not boot.  Power light is green, but AB are yellow with CD green.  So that should point to a memory issue. I reseated the RAM, no diference.  Then I tried the 2nd slot, still AB yellow..  I removed the RAM and tried a known good stick of RAM, same thing.  Removed CMOS battery and CMOS jumper and it booted up fine, all green lights. System worked great for 3 or so weeks until it went back to AB yellow but this time the power light would not light up at all.  So this time I did all the same stuff, but nothing worked.  Tried new power supply (yes it was a Dell PSU), still AB yellow.  Then the fun really began.  Ordered and installed replacement 2400 motherboard. But it was DOA, amber power light and only the D (amber) diagnostic light would power up.  Just to be sure I tried another CPU in the old motherboard, still AB yellow.  So I ordered and installed 3rd motherboard, same issue AB yellow power light out. Removed CMOS battery and jumper and hey it finally booted up.  Tested system out by powering on and off and quess what? Problem came back, AB yellow no system power light. Once again removed CMOS battery and CMOS jumper and it will power up fine again but only last a few boots then goes back to AB yellow. I have unplugged everything non essential for booting up the pc (hard drive, cd drive(s), PCI modem) didnt help.

 

Is this a issue with the 2400 motheboard? Or do I just have bad luck? what am I missing?