I have a lot idea.
How if you give us more info about your devices first (people has asking above, I'm just adding some).
Let's start with:

Motherboard type and series then,
PSUs wattage (Brands, Efficiency rates if possible),
OS you are using,
Driver version,
Powering scheme.

As much as you can tell us.

Have you tried with  2 X R9 on that mobo (380x + 390)? What else have you tried? I suspected that was less power and or driver issue.
If you tell us more info, we can help you more.