Unfortunately all I can offer to this conversation is that I'm having a similar problem! I've removed the hard drive and tried booting from a USB drive, I've also tried the different RAM cards individually in case there was an issue with it. I even tried removing the video card and tried to get it to boot from a USB drive with an SSH connection but it never gets that far.
The machine was working fine before so the last thing left is that there's a hardware issue with the motherboard. Unfortunately I don't have a second motherboard to test that assumption.