Want to drop my 2 cents here.
Once I had massive issues with X-Plane where it would crash (blue screen) only when on X-Plane. I know it’s different sim but what I did was run the Intel processor diagnostic tool and found my CPU was at fault and failed the tests most of the time. Got it replaced (it was an i7 9th gen K) and its been perfect for years now.
BTW, Before I had tested with that tool I had noticed if I turned off turbo in the bios, it would never crash, which lead me to believe it was a faulty chip.
Since you say you never crash / shutdown while not in MSFS what Im about to say may not apply to you but I’ll give the anecdote anyway: One time on a friends PC where it would randomly shutdown we found it was a faulty power button on the case itself. We replaced the button and harness that goes to motherboard and fixed the issue. The funny part (you sometimes have to laugh at your misfortunes) is we figured that out only after replacing everything else on that PC. The point is that sometimes the culprits are less obvious.
Hope that helps. I know the frustration!