What such an amazing graphics card you got, Radeon RX 470 GAMING X 4G | MSI Global | Graphics card - The world leader in display performance ! This card could decode H265! Please pay attention that this card requires PCI-E 3.0 as the connection, but your motherboard, ASRock > 970M Pro3 , could only support PCI-E up to 2.0. You RX 470 would work in downgraded mode. Would you please update the mobo firmware, ASRock > 970M Pro3 , to the latest. And have another test!
This problems mostly occur when the software requires the device working in its most optimal mode, when compatibility problem occurs, system would halt. If possible, try it on a motherboard supporting PCI-E 3.0.
Caution: Updating firmware is very dangerous, wrong operations would damage your system permanently.
install the asrock update, it can find all the needed BIOS and other device drivers fast