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.
I had the same issue and found the same fix on another forum. My main issue now is getting a good hash to power ratio. Without applying any soft power table mods I can get 1600 H/s at total system power draw of 200w at the wall for one Vega 64 (idle power draw is about 55w). That's with two threads in xmr-stak (2016 and 1600 intensity), -50% power limit, -30% GPU, and 900 MHz on the ram. Can anyone please advise whether this is a good hash to power ratio or whether I can do better?