I originally thought that I had an issue with my iMac 27inch retina 5k late 2015.
The problem was that my monitor started flickering / flashing every time I started (after 5 to 10 minutes of use) editing in Lightroom Classic in Creative Cloud. It has been doing this for the last 3 weeks. I was assuming that my graphics card was on its way out.
I've contacted Apple for assistance to which they couldn't figure it out. I ended up having to take it in for them to diagnose the problem.
I've collected my iMac today to be told that my iMac is perfectly fine and they suggested that it's a software issue, directing the problem towards Adobe.
Has anyone experienced this and could recommend a fix?
The OS I'm using is Mojave (which I've been using for months).
It is most likely a corrupted or out-of-date graphics driver. I would re-install the graphics driver. If that doesn't help, there may be some hardware malfunction. There is no feature in Lightroom Classic that will cause a screen to flicker or blink.
I hate to tell you this but you have a Hardware problem. More than likel the video card is overheating and or failing.
Of course Apple is telling you it is Fine. Your Mac is at its End Of Life. Apple does not support Mac computers older than 5 years. Even if they would fix it the cost would be near what a New 27" iMac would cost. To change the video card you have to change the complete motherboard (Apple Speak Logic Board) as the video card and at least Main Intel CPU are Soldered to the motherboard.
In LrC turn off the option to use the GPU and see if the flickering stops.
Get some compressed air and Blow out the dust that has collected in your Mac over the 5 years you have had it. That might help keep it cool and or Cooler.