Maybe u get a proper answer if u give computer specs' and maybe Cpu temperatures, voltages. More data ...more chance to get help.
As a fun tip... run your computer not using the classic power button,on the case, instead just use a screw driver to power up ( a short touch on the power wires , on the motherboard
). Or maybe u have a mobo with power button on PCB. U know... sometimes, when u have such behavior it may be just a gone-bad power button.