I have the same problem on 4 rigs with 4 various Vega 56 cards (ref with samsun g and asus/powercolor with hynix, with timings and without timings ). The card runs for an hour or two and then the hash rate drops to zero or to 1400 h/s with a DEAD message. I have observed in several rigs, but for some reason problems always with the first PCI slot (PCIe 3:0.0). If you do not stick the card into the first PCI slot (PCI 16x), then the problem disappears. Now I just do not use 16x pci slot