I don't know which GPU you got. But I am assuming its either an RX 470 8GB or an RX 470 8GB.
Your speed is pretty high and I am assuming you are running into stability issues with certain GPUs, like the ones that crashed.
Lower your clocks and increase your voltage a little and see if the problem goes away.
I using SAPPHIRE RX580 nitro+ Special Edition 8GB
Yeah actually, it seems like all of your GPUs are overclocked too much. Toning down the OC setting would really do a lot to help you with stability, as well as with the lifespan of your cards. As a guide, you are currently doing 31.4 MH/s per GPU. Toning them dow to 31.0-31.1 MH/s flat could be a good target to increase stability.