Tesla S1070 halts or hangs with two pci-e x8 interfaces

Have a big problem with Tesla S1070 with PCI-E x8 interface cards.

With only single card it works ok. Any combination of interface cards, cables, slots.

But when i connect with two cards it halts system or hangs just few secounds after computation started.

Tried following

GA-Z68A-3DH-B3 motherboard. It have 3 PCI-E x16 connectors, one in x4 mode, two in x16 or x8 mode, depending on both populated or first only.

Well, probably those x8 mode is somehow incorrectly routed on motherboard, so i tried

Asrock Z68 Extreme7 motherboard with 5 PCI-E x16 with special chip to warrant enough lanes, one PCI-E in x4 mode, one x8, and two x16

received following result

only one card is visible after boot, and only in sli preffered slots both visible but hangs system when calculation starts.

Can someone suggest something?