I have a triple-monitor system using an RX 480. The recent drivers - 18.7.x, 18.8.x, 18.9.x - all cause constant visual stuttering in every game using Unity when I play them on a single monitor and turn off my side monitors.
It's not an Eyefinity issue, as the monitors are simply expanded via Windows. The issue goes away if I turn on one or both of my side monitors, and reappears the moment I turn both off. Audio is not affected, but visual output is.
Testing in one game - Forgotton Anne:
With one or both side monitors on I get a constant 20% GPU utilization, constant 60fps, and steady 17.8-18ms frametimes.
With both side monitors off the GPU utilization spikes constantly between 12 and 19%, the framerate spikes constantly between 38 and 60, and the frametimes spike constantly between 18.2ms and 385ms. It renders the game unplayable.
I've been able to reproduce this, and the above results, with every Unity game I've tried. As soon as I turn off the second side monitor, the GPU utilization starts spiking, as does the framerate and frametime.
Games are configured to run Full Screen at 1920x1080, but I've tried Windowed and that's even worse. When Full Screen, only the game stutters, and alt-tabbing out or turning on a side-monitor stabilizes it. But in Windowed mode, if I tab or click out of the game without first turning on a side-monitor to stabilize the game, all of Windows stutters until I tab back into the game with a side-monitor on to stabilize it.
It might be a power-saving glitch. I've tried setting every power state to full speed, but that hasn't resolved the issue.
Driver 18.5.1 and every version previous worked perfectly. Something was broken in 18.7.1, and hasn't been fixed since.
Win 10 64bit
Asus Z170-A, Bios 3007
RX 480 4GB - Driver 18.9.1 currently, but same behaviour observed with 18.8.1 and 18.7.1
Seasonic SS-660XP 660W Platinum
Dell U2312HM 1920x1080 @60Hz via DisplayPort
Dell U2312HM 1920x1080 @60Hz via DisplayPort - main screen
Dell U2312HM 1920x1080 @60Hz via DVI