The computer repair madness continues.
Just in case this saga goes on for much much longer => #pcrepairsaga
Tested all the power supply points for both ATX and ATX_12v cables. Impeccable and well within tolerance.
So... RAM after all? Hope it's not CPU :/
@yarmo Even if you test the voltage levels, that does not rule out a faulty power supply. The easiest route is to replace it for checking. It is much more cheaper than RAM.
@aluaces thanks for this insight!