I just built a new PC with a Gigabyte VEGA 64 - My PC would black screen and lock up under stress. What fixed mine was - In Wattman (Radeon Settings), Set the profile to "Custom" then manually adjust the fan curve (I put something like 40deg = 40% speed, 50deg = 50% speed etc. - you can fine tune it later) also put the power limit up between 25-50%. At stock settings the fans will only go to 50% of their potential speed. For my card the memory was overheating and the fans at 50% could only do so much. Since changing this I haven't had any major issues apart from the odd flicker when the v-sync / freesync settings are fudged because of my own doing.
Hope that helps - Mine needed a complete fresh driver install as the temp sensors were randomly spiking to like 3000 degrees!