Do not use mainline kernels and point releases Mesa. They work randomly as you see because of partially implemented drivers. Use the kernel below and Mesa dev git. Debian testing/sid Xfce is easier and more stable than Arch Linux with never ready buggy desktops kde and gnome. Oibaf Mesa git ppa bionic version is compatible with Debian testing/sid. https://cgit.freedesktop.org/~agd5f/linux/log/?h=drm-next-4.17-wip No problems with my RX560 with latest code.