Use MSI Afterburner and raise memclock to 1000MHz, leaving first time core clock and voltage stock, then try to reduce core voltage and increase core clock too, HBM mem is dependant of temperature and core clock too.

Then when U got stability, can try to increase memclock above 1000MHz.
https://www.guru3d.com/files-details/msi-afterburner-beta-download.html

But better, sell Vega 64 and buy RX5700XT, U can achieve 52MH for around 103-110Watts GPU (around 130 Watts entire graphic card).