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To test or use the PSU separately from the motherboard, you have to put in contact together the Green wire of the 20/24 Pins ATX plug with the Black wire which is immediately close to the green wire. I have just watched the plug of a recently dead PSU I have still here, and there is a black wire on each side of the Green one. As all black wires are 0V Common wires, each of the two should do the trick.

Thanks roubal, in fact this is exactly the same config I have, with the câble at this place.
But with this, as you said, there is no link with the mobo...

So I will buy a "double psu cable" commonly used for crypto mining

I have at least 750W power supply, but see the same problem since about 1.5. I'm wondering about software issues - either the Nvidia drivers or Octane. I have a GTX 770 that only gets to about 500M to 700M of GPU memory, before it fails.

But with this, as you said, there is no link with the mobo...

So I will buy a "double psu cable" commonly used for crypto mining

Hmm, I am not sure. The purpose of this kind of cable is probably to feed two motherboards with one PSU. Don't connect two PSUs on one mobo. IMHO there is a risk for both the Mobo and the PSUs : If one PSU fails or gives a slightly different tension the current may travel through the mobo in unpredictable way.

If you shunted the green wire on the second PSU, things look OK. Maybe there would be some things more to know about the Power OK wire on pin 8 of the 20/24 plug. I don't know if it is just for a LED or if there is a monitoring function on the motherboard related to this wire.

Roubal, I will try anyway, because its à solution...
About the double psu system, the second psu have just 2 cables linked to the mobo and I think its to have feedback of it!
Dont worry, all the rest of the power will be used by gpu.

Really, i think that what you said firstly makes senses and anyway, this is the only one solution to have a link between mobo and psu...

I will see the result next week and give a feedback, I'm travelling now.
Thank you for the support and help

I had a similar problem years ago.
Always when I switched to PT the system shut down. It was because PT consumed more Power then Direct Lighting.
In the end it was because the powerlanes have not been connected in a efficient way.
Try to connect the power cord connectors of the psu to different plugs (on the psu).

Try to connect the power cord connectors of the psu to different plugs (on the psu).

It is important if the PSU has several 12V rails. Often the power per rail is not enough for a recent GPU, while not all the power of each rail is used by other peripherals.

Modern monorail PSUs allow much more flexible power distribution with no waste.