I opened up the chassis and blew out all the sinks with an air compressor.
Although, I did not try to remove the heat sinks themselves.
It is possible that the thermal paste has dried up/ developed a gap.
The *nice* shutdown, not a sudden power-off.
"Windows is shutting down"
Happens without warning, out of nowhere.
One moment you're doing stuff, the next moment you're looking at the windows goodbye...