Hey guys,
I had exactly the same problem, but my PSU has 1600 W + there was no overclocking. Somewhere I have read about asymmetric RAM setups on the motherboard, and this was the reason in my case: I have 8 RAM slots (4 left + 4 right), and the RAM modules were not symmetrically distributed (e.g. 2 slots used right + 4 slots used left). After I have removed 2 RAM modules, so the slots left and right were symmetrically used, the problem of the spontaneous shutdown while MSFS 2020 was running disappears.
So my tip: look at your motherboard to be sure to have a symmetrical RAM modules distribution over the slots on the motherboard.