I imagine you would get more activity in your thread if you posted your Computer information, Hardware and Software installed.
Plus what errors are you getting if any?
What is the temperature of your GPU card when idle and when under heavy loads? Sometimes overheating will throttle the GPU Card.
Are you using AMD Wattman to configure your Vega64 Settings or some other 3rd party software like MSI Afterburner?
Have you tried to open a Sapphire Support or AMD Support ticket to see what they suggest?
Here is AMD Support link to open a ticket if no one else is able to answer your post: Online Service Request | AMD
