Just wanted to share some improvement I found.  I was having this issue where every 20 minutes to a few hours or so my hashrates would drop from 1800-1900 to 1500-1600.  I would go check the rig or login via teamviewer and simply restarting xmr-stak-amd would restore my hashrate.  This was driving me crazy as I hated losing that 600h/s across my two vega's.  My monitor was plugged in to the integrated graphics port on the motherboard and windows was putting the monitor to sleep after inactivity and I figured out this sleep mode was dropping the hashrate on my vega's.  Turning off the display sleep has preserved my hashrate so far.  I'm going to order a dummy plug to keep my hashrates up without having a monitor connected.