I recently upgraded my GPU from a RX 580 to a Vega 64 and I haven't had any issues while gaming or watching movies, or anything that requires a graphical load.
The only issues I have been having are when the system has been idling for a while - let's say any random time from 20 minutes to a few hours, the display will time out and at some point the system will crashed. The only way to recover it is by pressing the reset button on the PC. Pressing control-alt-del won't do anything as the system is locked up.
Checking Windows 10 event viewer does not show any errors or software that may have crashed. The only thing I notice is that when I log into Windows 10 it says that my Wattman settings have been restored to default. I monitor GPU temperatures in HWinfo and the temperatures are quite cool, nothing near values that would cause problems.
Things I have already tried but have NOT worked:
Doing a clean GPU driver install using DDU and/or AMD's clean install utility
Installing Monitor drivers
trying both HDMI and DisplayPort - the crashes happen with both
setting a TdrDelay value win Windows 10 registry to 8 seconds
setting the minimum HBM state to state 1 instead of state 0
disabling sleep modes in Windows 10
disabling Link State Power Management in Win 10 power plan settings
removing any software such as MSI Afterburner
setting Voltage Swing to 1 in Wattman. I have not tried setting it to 2 yet as I do not exactly know what this setting does and I don't want to damage my monitor.
It seems like there is an issue where the system goes into an idle state and then tries to wake the display back up. Some how it is crashing the whole system.