This is the issue that I have mentioned previously.
Happens on both Win 7 & Win 10, regardless of settings and resolution.
(I thought I'd solved it by setting everything to 1920x1080, but it just bought a little time)
AMD R9 390
16GB Ram
i7-4770K
LG 4K Freesync Monitor
*Possibly Fixed*
Suspect it was CPU temperature related.
Gave everything a good clean and it's going OK now.