I recently set up a new GPU (Radeon RX 480 Nitro+ 4gb) and faced some problems. When I play, all runs smooth untill the moment when my GPU heats up to about 50 degrees, then the image start to stutter (every 1-2 seconds), when the temperature drops below 50 everything is smooth and stable again. It doesn't depend on graphic settings: game runs smooth on low and high quality settings untill the GPU heats up to 50. I know it might not be the cause of my problem but there can be a correlation between these factors.
It happens in any game I tested: Fallout 4, Crysis, Witcher 3, GTA V, Overwatch etc.