Running Outlook on the iMac causes the screen to flicker and eventually black out and reduce to half/split screen display.
If I don't run the Outlook app the Mac runs like clockwork.
With Outlook active, at some point the flickering begins and then gets progressively worse until the screen is completed fuzzed or it pops back to a half black screen (which continues to flicker intermittently - more often than not)
Whilst it sounds like a hardware issue (and I've ordered a new Mac thinking it was!) it appears to be tied to Outlook.
iMac (Retina 5K, 27-inch, Late 2015)
3.3 GHz Intel Core i5
8 GB 1867 MHz DDR3
AMD Radeon R9 M395 2048 MB
Mac OS 10.14.1 (18B75)
Rebooting seems to clear it until we run Outlook again, then the cycle continues.