What is wrong with my new build?

[omnipresent]

tldr; My computer powers on and everything runs for 10 seconds, powers down for 4 seconds, then powers back on at which point it stays on until I shut it off. Throughout this, the computer does not POST and there is no display from neither graphics card nor onboard connection. Do you think my motherboard is at fault?

So I had built a pc once before, no problems. The other day I built my second and everything went smoothly, or so I thought. Turns out when I pressed the power button, things weren't right.
So what happened was everything seemed to be getting power for ten or so seconds, then it all shut off for about 3 seconds at which time it turned back on and all of the fans continue to run fine until I turn the computer off. But I do not get any display to my monitor. I was distraught as I was not expecting this hiccup, but since then I have been reading up on and learning a lot of stuff about computers which I think has led me to my conclusion about the problem.

So here is my testing:
1. All components connected - computer powers on, off, then back on, no display.
2. Detached graphics card, connected hdmi directly to motherboard - same result.
3. Tried removing/replacing RAM - same result.
4. So then I discovered and tried breadboarding - same result.
5. At this point i researched some more and learned about the POST beep (and the little case speaker's function in debugging problems). So I plugged in the speaker and - same result, no beeping.
6. I took some advice from online and removed the RAM before powering on - same result, except this time the speaker beeped three times which I learned was the proper response to the motherboard not having any RAM in it.
7. So at this point I thought it must be the power supply which was at fault, but I took a PSU from some old computer my parents discarded, and I got the same result -- if the computer does that weird boot thing, with two different PSUs I figure they must not be the problem.
8. I tried removing and replacing the CPU - same result.

So this leaves me with two possible culprits:
1. The CPU - I don't think it's the problem but I'm not sure. My reasoning was the fact that when I breadboarded without the RAM, the speaker made three beeps indicating that there was no RAM. I figured that if there was a problem with the CPU, that would take precedence and the speaker would be appropriately silent.
2. The motherboard.

If you have gotten this far, thank you for taking the time. My question is, does it seem like the motherboard is the problem? I would like to buy a new motherboard to find out, but I am worried that it won't solve my problem and then I will be stuck unable to return it. I would then be left assuming that it was the CPU at fault?

Thanks for any help!

 

[cin19]

How about try clear the CMOS by the jumper or removing the battery, I don't see you try it yet. After clear the CMOS, boot the PC, I guess you maybe ocing the PC, something else, and it is unstable or not compatible to cause the boot/reboot. Also you got three beeps when no RAM, that means the CPU/MB are fine, if there is not bent pin(s) Note: for intel the pins are on MB, AMD they are on cpu.