I can't type below a table once it's pasted in. Due to the description of 2,3,4, I think you have it narrowed down to the MB. Either one resets CMOS - CMOS jumper or replacing the battery. [URL] 
Hello, I have a problem with my Dell Optiplex 780 which I've already described. My problem is a bit different than others, because the PC booted up once and then died again. I plugged in only power and VGA, replaced half of my PC (PSU, CPU, RAM, CMOS battery) and it's still dead. Is there anything I can do other than throwing my current motherboard away and buying a new one? Can I somehow reflash BIOS if that's the issue? I don't even know what's wrong with this machine at this point, especially that it worked for a while.
Dell Optiplex 780 back to dead after booting up once, 234 blinking and solid 
amber