Hello, i have a similar problem, also sapphire vega 56,
crashes when having overwatch (1080p) or other games in a window, and doing surfing or watching youtube on the other side of the screen (4k)
but when i play games in fullscreen @4k or just surf its fine.
I have a similar problem too. I have a MSI Radeon Rx Vega 56.
It crashes in almost every game, lag or black screens. When the screen turns black, I can still hear the sound from the game normaly. All 8 dots on the graphic card is turned on and stays that way.
CPU - AMD Ryzen 5 1600
Ram - Corsair Vengeance LPX 8GB DDR4 Black 2666MHz
Motherboard - Gigabyte AB350-Gaming 3
Graphics - MSI Radeon RX Vega 56