Does the motherboard have some sort of automatic SLI capability? If so turn it off.
Not that I could see in the BIOS settings
Turn off your integrated graphics in the bios and just use the 1070ti in the primary x16 PCIe slot. If that's stable, add the next 1070ti to the next x16 PCIe slot. If that's stable, then add the next 1070ti to the first x1 PCIe slot.