I had the same problem with Xubuntu 17.10 and maybe 18.04 (can't remember). I 
GPU would hang when watching Videos with mpv or even in Firefox. When I tested
gnome-shell this would also sporadically hang.

What it solved it for me was to switch from Radeonsi to Amdgpu.
(add radeon.si_support=0 radeon.cik_support=0 amdgpu.si_support=1
amdgpu.cik_support=1 to your grub.cfg kernel boot parameters) 

Now I've upgraded to 18.10 and decided to give radensi another try (mainly
because VLC refuses to deinterlace mpeg2 under amdgpu for some reason) and it
works without problems for about ~2 weeks daily use.

MSI R7 370 4G
FX 8320
Gigabyte GA-970A-DS3P FX