I had this issue but it went away when Windows installed the Creators update. I thought I'd got Windows all up to date before hand but that update seems to work differenetly like a whole new Windows version.
For me it was just windows updates! I thought I had installed all the updates but then windows popped up to install a big update that looked like a new version of windows (possibly the Creators Update?) It took ages but as soon as it was done my Vega worked fine.
I'm having the same issue with an MSI a320m mobo. Clean install, bios flashed to latest and late chipset drivers. It wont be power related but I dont yet know the cause. I'll update here when I find it unless any one else does first.