My rig (6 cards) immediately crashes when I start mining ETH. When I take one random card out it mines stable on 5 cards. It works perfectly on all other coins with 6 cards. I've read in old topics that it could be the PSU but I have a new Corsair 1200W for 6 RX580's so that should be enough. Any way I could test it?
use the command -lidag 2 and see if that fixes it. There is an inrush of power at the DAG generation phase, and your power supply may be too sensitive to it. By slowing the DAG generation just a bit your power supply may be able to keep up. I had this same issue on a couple rigs and this fixed it.