One to keep in the back of your mind:

Had a system with intermittent shutdown or rapid shut-off at start. Appeared to be motherboard as it could happen with just board & CPU.

Turned out to be a blob of hot-melt glue that had been positioned across the back of the soft-power switch to secure it. It was contacting the terminals where the wires attached, and had developed current leakage that at times made the motherboard think the power button was being pressed or held. Removing the blob of glue cured it.