If I turn on my PC with the case button (and, for that matter, the 'start' button on the mobo) the PC begins normally.
If I plug my PC into the wall directly, the PC turns on by itself and the BIOS resets. I don't think it's a dying CMOS battery as the BIOS settings are kept if the PCs turned off and on using the power switch.
I've posted this in power supplies as it's obviously to do with power. Really though, I've no idea where to post it.
I'll pop out later and get a new CMOS battery. Give that a try.
@rdc86 - excuse the poor explanation. I live in The Philippines where the wall sockets and plugs look like this
i.e. as soon as the plug is put into the socket, there's power coming from it.
If I plug the PC into the wall / powerstrip etc, the PC turns on and the CMOS resets without me touching the power buttons on the actual PC tower.
If I turn on the PC using the power buttons on the mobo or PC tower, it begins normally and the BIOS settings are as I left them.
Hope that made sense.
I'll post back if a new CMOS battery solves the problems. I'd understand that the CMOS battery being dead explains the loss of BIOS settings, but could it be responsible for the PC turning on without me touching the power switch?
I think it must be the motherboard. It's not shorting as it did exactly the same built up out of the case and with only my SSD, keyboard, GPU and monitor connected. I did it with an old GPU (that runs fine in another PC) and a different PSU.