Found the issue while check if any video cable was bad. The GPU got very hot. As the drivers are not collecting temperature sensor from GPU, it was not visible for me. Putting all FAN on full power all the time was enough to keep GPU cold, no "power" fluctuation or computer crashes. As almost always, the problem is on the basics.
Thank you all.
Funny that they said it would require a complete architecture change to be able to. What a mess