could be a power supply issue
I had similar issues on my wife's setup when she ran
3700x, 2x16GB 3200, ASRock B550m Phantom Gaming 4, RX 550 2GB, Thermaltake RGB Smart 500wtt 80 Plus (white rated) PSU
I thought it was a GPU issue .. replaced the power supply with a Segotep 600wtt 80+ Gold rated one and issues went away
The GTX 970 isn't as power hungry as the R9 380 4GB