This issue has been discussed multiple times. Heres a recap:
Shirty AMD drivers...HBM craps out any time you remove a monitor or power save kicks in and various other undocumented reasons.

Solution:
Don't install Wattman
Get HDMI dongles
Use a hashrate monitoring script to
- Disable GPUs
- Enable GPUs
- Set speed etc via OverdriveNtool
- start Miner
- monitor hashrate

Script can be found here along with many other tips and tweaks:
http://vega.miningguides.com/#Script