Still plagued by the annoying screen flicker on your precious 27-inch iMac, even after the previous update? Try this second attempt by Apple. While you're at it, there's also a minor update for iTunes which makes sure it actually "remember[s] password for purchases," as well as fixing a few sync and performance issues. Good luck with both and let us know if your iMac nightmare is finally over.
AppleiMac 27-inch with Retina 5K display (late 2014)