Set min fan speed to 3000.
It is droping cuz of HBM temp and the fan rpm is related to core temp ONLY.

So, HBM must be bellow 70 witch means core temp must be bellow 60 since HBM is usually 10 hotter than core. You'll need to check that.

If you was getting 2k, its not windows or bios. It is temp.

You dont need to follow the guide but there is a lot of info there. Check windows settings and crossfire in registry.
http://vega.miningguides.com/p/tuning.html

good luck