HI,
I also have hard time with Vega64 (gigabyte gaming oc/samsung mem). Can't get it past 40mh no matter what.
When ever I try to change timings (even tRCDRD and tREF) I usually get BSOD. Just overclocking not higher then 990@900-950mV or crash. Then tried to flash to vega56 and with this I can't even get to stock clock 945mhz before BSOD. Also temps Are 50-55 core and memory max 75c. pwr supply 1300w almost triple the needed and with separate cables not splliter. On windows/phoenix miner. Also repasted with good paste, thermal pads are most likely still oem.
I bought it used...maybe it is just faulty or big time looser in silicone lottery?