Computer will not POST after shutdown

I am having an issue getting my motherboard to POST. This has occured pretty much since I purchased the pc about 2 months ago. When I can get it to boot up into Windows it works fine and doesn't turn off or restart at all, no issues. If I shutdown the pc or if the power plug is disconnected the pc refuses to POST upon booting again, but restarting it from windows menu doesn't cause the issue. The power on LED comes on, I can hear the hard drive, and all fans spin at high speed but, no display input or power to mouse/keyboard. It then turns off after about 15 seconds on it's own, then powers back on again without POST and it stays on for good with the same symptoms I just desctibed (no display signal or keyboard/mouse power from usb). I've been resetting the bios and leaving it unplugged for a little bit to get it to POST again in the past and it would work sporadically. Recently though, I put it in hibernate overnight and since then nothing is getting it to POST. I've tried leaving it unplugged overnight with the cmos battery removed because it would work again by doing it in the past. Even replaced the cmos battery with a new one. I've done all the possible troubleshooting imaginable for every peice of hardware except trying a different psu and motherboard. My question is, will a psu or mobo slowly cause this issue or could it be bios related or something else?
Any info would be appreciated. Thanks!