Even at 870mV, I am still unable to get a stable system running. Dead GPUs and hashrate drop to 15kH/s still occurs after several tens of minutes of mining. And the weird thing is hashrate drops happens to Hynix mem GPUs only.
I can confirm the same behaviour on Vega 64 with samsung memory at L28+28, tried droping to L26+26 and still same results.
Normal : 19.56, and after some random time 19.06