GTX 780 won't show up in Win 7 + S7025

hello,
following my other thread, I decided to open another thread to find out if somebody else have been able to get an GTX 780 working
MB : S7025
BIOS : 1.07
1 Nvidia Quadro FX 5800
2 x asus GTX 780 Direct CII
1 x asus GTX 670 DirectCII
PSU : 1500 W
Win 7 pro 64 bit

The Following setup work:
Quadro Alone
Quadro + 670
670 Alone

The following doesn't work
Single GTX 780 (on any slot)
Quadro + GTX 780 (on any slot)

I've got 300w per rail, so it's plenty enough.
The leds on the GTX 780 are green
The slot are PCI-E 16x
The GTX 780 is not visible at all even not in the device managment

I don't know what elese to test to make it working. May be the GTX 780 are faulty, who knows!!!

 

Just Tried the GTX780 on a very old Dell 490 Under win 8. It works immediatly!
So I guess the problem is link to the MB.

 

It is very weird, but I finally found out :
- On board Vga must be set to Enable in the bios to get a single GTX working
- A second Monitor have to be attached to the second card to get the card visible in the OS
- Same for the third card.
- My slot (PCIe) N° 5 is out of order.