My second screen flickers to black every now and then. It seems to be dependent on what is being drawn in it, because if I just have the desktop open, then it won't flicker very much, but if I have a program in it, it will flicker a lot, and if I have a youtube video playing in firefox on that screen, it will flicker so much that it's off about 85% of the time, and it becomes unusable until I can get firefox onto the main screen. I have used 3 graphics cards in my computer since it started: Radeon R7 260X, AMD RX 550, and AMD RX 560. The second screen is in portrait mode, not landscape. The screen works perfectly fine on Windows, so I know it has something to do with my drivers on Linux. I looked at https://ubuntuforums.org/showthread.php?t=2061180 and it looks most like my problem, except I'm on AMD instead of NVIDIA.
cat /etc/lsb-release outputs
DISTRIB_ID=Ubuntu DISTRIB_RELEASE=16.04 DISTRIB_CODENAME=xenial DISTRIB_DESCRIPTION="Ubuntu 16.04.3 LTS"
uname -a outputs
Linux [my hostname lol] 4.13.0-26-generic #29~16.04.2-Ubuntu SMP Tue Jan 9 22:00:44 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
lspci -nnk | grep -iA2 vga outputs
01:00.0 VGA compatible controller : Advanced Micro Devices, Inc. [AMD/ATI] Device [1002:67ff] (rev cf) Subsystem: Micro-Star International Co., Ltd. [MSI] Device [1462:8a91] Kernel driver in use: amdgpu
env | grep DESK outputs
DESKTOP_SESSION=ubuntu GNOME_DESKTOP_SESSION_ID=this-is-deprecated XDG_SESSION_DESKTOP=ubuntu XDG_CURRENT_DESKTOP=Unity