I have now run ~72 hours w/o any issues with 24 Vega64 GPUs(good and bad ones) mixed in 4 separate rigs ... CNR with core 1407Mhz /860-970mV , Mem 1080-1100Mhz/860-970mV.
I would say this is pretty stable setup. Pool side 24h avg hashrate fluctuates in the range 54,1-54,4 kH/s
--CL 19 --RAS 28 --RCDRD 12 --RCDWR 5 --RC 44 --RP 12 --RRDS 3 --RRDL 3 --REF 15600 --RFC 250
I am happy and have in my mind... "dont touch when things go smoothly".
Thats an insane voltage range, and I cant fathom how 970mv could possibly be necessary. Between 9 64s and a flashed 56, I run avg 834mv, none over 850, for similar clocks/timings. That puts me @ 178w per GPU at the wall.
At 970, youre running at least 35% higher power, meaning over 240w per card.
It sounds like you have some ppt issues or something, maybe youre running your SOC @ 1200mhz unnecessarily (it should be 1107 @ your clocks.) Although, even then, you prob shouldnt need much more than 925mv.