mabye this gpus are not stable for mining [..]
Mining with three cards and avoiding one slot (pcie bus, riser, riser energy, gpu energy) seems to be stable. But I'll try to use Windows 8/10. May the nvidia driver is crap (it is, but the question how much).
Well there is something wrong with the slot then you know this because you used 3 different working gpus. Does that slot work on its own with just one gpu which is itself ? If so then that is very strange. I don't think that is the problem though. I think the problem is the slots broken as you say. Try another motherboard of you can. I think you going to need to buy another motherboard though.