Question Problem with computer failing to boot for couple hours

[Avuee]

My computer turns off randomly (Even with UPS installed) then continuously reboot for 2 seconds before turning itself off. I only hear fans and HDDs starting then PC turns off like it lost power

I just can't figure out what's causing this problem, this can't be power problem since I installed UPS, and it starts working normally after leaving it unplugged for couple hours. I tried swapping RAM and removed Hard drives but still no improvement

I'm suspecting this is either motherboard or CPU problem, but I can't figure out which part is causing problem so I hesistant to swap my parts

Can anyone help me with this bizarre situation?

 

[rgd1101]

full system spec? include make and model of the psu. and it work without the ups?