Z170XP-SLI systematic reboot when starting OS installation

[cc_rock]

I just built a new PC with these parts:
- Z170XP-SLI (bios upgraded to F6, optimal defaults, no overclock)
- i5 6600K
- Corsair Vengeance DDR4 memory 2 x 8gb
- SSD Sandisk 480 Gb (AHCI)
- 1000W PSU (novatech)
- no discrete GPU, using the integrated one

The problem is very strange: the system boots up, goes past POST phase and the OS installation starts (I tried Win 10 and Ubuntu 16.04 and the behaviour is the same) and after about 10 seconds of showing the OS initial logo, when the screen is about to change resolution (I guess), the system powers off and on again, and reboots.
I am booting from USB stick, no optical drive.. I have an usb keyboard and no mouse.
But the most important information is that if I install a dedicated GPU and connect the monitor to it, all works fine.. I am able to install windows and use the PC normally.. so the problem seems related to the integrated GPU, somehow.

Any ideas?

Thank you very much!

[cc_rock]

I can add some more information: While keeping the discrete gpu connected, I turned on the integrated one and tested it in Windows (monitor connected to motherboard of course), and it works properly. While the other card is connected, I can even do the full boot on the integrated gpu only, without problems.. But as soon as I disconnect the discrete card, the problem arises again and the pc reboots right after the os starts loading.. So now I'm thinking about two possibilities : a bios bug that does not initialise the igpu correctly if there's nothing in the pci slots (weird), or more probably a power problem.. Maybe my 1000w psu has too low load without the pci card and does not work well at that low wattage.. What do you think?

[dmdilks]

Have you tried to update the bios it does have a problem with some video game using integrated gpu. I don't know if it would fix it. But if it doesn't just get yourself a really cheap video card. You have way more power then you need. Plus any cheap video card is really better then the integrated gpu.

[ElectroStingz]

Hello,

Can you take the memory stick which is in the first slot (channel A) out, use only the last one and try like that.
If not sure try each stick individually with the integrated GPU.

Also make sure you are not running the XMP mode.