Re-connect absolutely everything, and make sure nothing loose inside. Mains lead often the culprit, ensure it's pushed hard into PSU. If it persists, may be a cracked joint on mobo, try flexing board gently whilst running to see if it shuts off.
this might sound silly but id consider opening up reconnecting everything and giving it a hoover out ! my was doing the opposite, turning itself on this solved it , guess there was a bit of dust caught someone creating issues .