The only way to be sure its the motherboard is to eliminate everything else first. That is why I always check the battery, adapter, jack, dc power board....Then I swap out the Ram. You can also try to boot into the bios with the hard drive removed to check that. Other than that, I just disassemble and reseat every component in the laptop and clean thoroughly. Components can become loose and excess dust can cause havoc. I remove the display, keyboard, video card, power board, and anything else that is not attached to the motherboard itself.
I once had a customer bring me a Dell laptop that had the same symptoms that your laptop has. I found out the she dropped it a few times, so I tore it down and found a loose board. I cleaned it all up and replaced the board securely and it booted right up into windows.
I hope some of this helps. If none of it does, then you may have a bad motherboard. I would look for signs of anything that appears to be burnt out. Other than that, not much I can offer without having the computer in my possession.
Good Luck