I had the same issue appearing a few months ago, without any significant system change.
I swapped the (rather old) PSU to a (similarly old but) stronger one, and the issue went away. Strangely, the power-offs did usually not occur at full load, and the respective PSU is now driving the desktop without problems.
Lesson learned: these PSU do degrade over time (at least when running with serious load) and what was calculated to work and did once work, may suddenly start to produce random failures.
