I have a strange problem with this combo. On 3 cards in PCI-E 16x slots everything are working well. When I'm trying to add 4th card to any PCI-E 1x slot, problems begin. System randomly start up. In first case, everything (sgminer too) is no problem without manual reboot. In second case, I have a black screen with white "_" and system freezes. On compatibility mode problem doesn't occur. Lspci | grep VGA show four cards in any case. In BIOS PCI-E Mode is changed to Gen 2.

Oh, and I'm using powered USB 3.0 Riser PCI-E 1x -> 16x.

My mining rig:

Core i5 3470 (for CPU mining)
Asus Z77 Deluxe
4 x Sapphire Vapor-X R9 280X
2 x Corsair CX750M with Add2PSU

I have to solve this problem, because mining rig will be remotely controlled from a distance and I would like to be able to reboot