Here's something that'll fix a bit of your 27-inch iMac ails. Apple just released a graphics firmware update 1.0. Order of the day? To "address issues that may cause image corruption or display flickering." Download away, and let us know if you troubled souls have seen some relief.
[Thanks to everyone who sent this in]
AppleiMac 27-inch with Retina 5K display (late 2014)