SOLVED!

I tried all your suggestions, but nothing worked.
Then I bought 2nd PCIe-to-4USB adapter, so now ALL risers are connected to those quad-USB adapters (not to single PCIe adapters).
And now it works like a charm!

Seems like my mobo MSI Z270-A Tomahawk is not capable of driving more than 3 GPUS directly via single-PCIe risers.
But Quad-riser did it's job. Weird, but it works.

Hi guyz!

I have a new mining rig (MSI Z270-A, 8x KFA2 1070Ti, 2x 850W PSU) and have problems running all 8 cards.
When all 8 GPUs are running, rig simply after some 10-20 minutes shuts-off or just stops mining, depending on OS I use.

Settings:
- Above 4G ON, PCIe Gen1 or 2
- No overclocking

Tried:
- With windows 10, SMOS, HiveOS linux...same story
- Exchanged risers...same story
- Changed CPU cooler and thermal paste ....same story
- Changed RAM ....same story
- Underclocking ....same story

Symptoms:
- all 8 GPUs .... stuts-off after 10-20 minutes or simply stops mining
- only 4 GPUs on one PSU    ....works OK
- only 4 GPUs on other PSU .... works OK
- only GPUs 1, 3, 5 and 7 on both PSUs ....works OK
- only GPUs 2, 4, 6 and 8 on both PSUs ....works OK

Any idea?

SOLVED! Finally
First I had to clone OS from M.2 disk to SATA SSD disk to free up PCIe lane. That was the catch.
Then I pulled out all GPUs, ONE BY ONE to determine, whether it is the catch on PCIe slot or on PCIe-to-USB adapter. And it was PCIe! Simply could not handle more than 3 GPUs directly attached to PCIe slots.
So I used:
- 3 GPUs via risers directly attached to mobo
- 1 GPU via M.2 PCIe adapter
- 4 GPUs via PCIe-to-USB adapter
Plugged in then 1-by-1, rebooted and WAITED few minutes after each reboot, until Win10 settled down. If GPU was not found, rebooted again. After 2 hours rebooting and plugging-in finally all 8 GPUs running

Thank you for suggestion on moving to Altcoins. My bad choice of forum.

Regarding Windows 10 build...it is 1709, patched to the today's  latest.
Nvidia drivers 391.01, also fresh.

Hmmm...I have SSD on M.2 slot. Might this one take the last PCIe line, thus not being able to run 8th GPU?

Hi,
I've build up mining rig on MSI Z270-A with 8x Nvidia 1070Ti GPUs. I stuffed some of GPUs into PCIe slots with risers, and the rest into PCIe-to-USB 4x riser.
But regardless of which GPU I stuff where, I can only get to 7 GPUs being recognized by Windows 10.

Updated Windows 10 to latest, patched nVidia drivers to latest, removed and reinstalled them fresh, removed all cards and tried different combinations, stuffed them in one-by-one into different slots, tried 5 risers + 3 in adapter, 3 risers and 4 in adapter and 1 to M.2 adapter, tried different PCIe slots...but still cannot get Win 10 to recognize 8th GPU.
And it's not particular GPU/riser/cable, but ANY GPU which I add as 8th is not recognized.

Is there anything I missed in BIOS?
Of course, I have "Above 4G" ENABLED, UEFI Enabled, Win 7 Disabled, HD Audio DISABLED, COM and LPT ports DISABLED.
I have also DISABLED integrated Intel graphics adapter. But still no joy.

Maybe anything to do with "PCI Latency timer" which can be set to 32, 64, 128, 196, 224?

Ideas welcome.