I recently built a system with a Ryzen 1700 and MSI RX 480 4G and have some absurdity in its graphics performance as well. While it does well on Doom, Fallout 4, and Tomb Raider, it struggles with Far Cry 4 and I have crappy frame drops in GTA V, especially at night, all with post FX, reflections and shadows turned down. It appears that the RX 480 can just be finicky at times and give such issues. I'm wondering what to do as well with those games, as a reinstall didn't help. Neither did a Windows reinstall or any amount of fidgeting with the graphics settings.
If you are running Windows 10, the Xbox DVR application is set to ON by default. Even if you don't or never created a Xbox account, it is still set to ON by default.
Here are steps on how to disable it:
When you perform a Windows 10 update or update to the Creators Edition, it may enable Xbox DVR again and you have to disable it again.
This app is known to cause FPS, stuttering/lag and FreeSync issues.