Despite you have replaced the motherboard, as others have said I can imagine a bad VRM taking the system down.
In a very distant past my main computer was a Dual Pentium II 300MHz and I had an issue that almost took my power supply with it: one of the MOSFETs of the Vcore voltage regulator failed short and was putting almost 3V into the CPU0, which in consequence started consuming an immense amount of power. The computer was still operating normally and I only noticed the Ampère smell coming from inside the computer.
At the time I was using a ~400W (untested) power supply and a Supermicro motherboard. They repaired the board for free - a feat given it was completely out of warranty and I had to ship it from Brazil to US.
Fast forward many years, I had a crap power supply and its mildly "out of mechanical tolerance" contacts inside the main connector almost ruined the motherboard receptacle - as wraper said, the minor sparks caused by the poor contacts started to melt the plastic due to the increased temperature. In this case, the symptoms were easier to notice: the computer randomly failed. Replacing the power supply with a high quality Corsair with detachable cables solved the problem. The POS is still in my junk storage, but it has a big red X on it.