Problem with iMac retina 5k

I have a new iMac 27" retina 5k, made late 2014 but bought in 2015, and it is gorgeous apart from one thing. After a while browsing graphic-heavy sites eg. newspaper sites, or editing photos, or anything else to do with photos, it starts flickering and flashing, and eventually the screen freezes with square tiles all over the picture. I then have to hard reboot, and sometimes it immediately does the same. Sometimes it even freezes while booting. It's getting worse, and is practically unusable, and I'm about to throw it out of the window. I've spend hours on Apple Chat with technicians at Colorado, and taken it to the Apple store, but they cannot replicate it and find nothing wrong with it. I showed them video and photos of the problem taken with an iPad. Apple blames third party apps or software. But I got nothing unusual on it. Once I found hundreds of files in Finder named Kernel_XXX type GPU_Restart, which is a clue, and I took a screenshot - lucky I did because they all disappeared like magic and no longer recur. Once it froze while I was running a video on VLC Player, and the sound continued - so it's a screen/graphics problem, not a whole system crash.

I cannot find anyone else in the world with the same problem, and Apple claims it is unique. Any ideas?

Purely based upon your description of the fault it's likely you have a GPU related hardware fault, and if the fault is intermittent it can be a pain to confirm. If it is, then you'll need a replacement logic board, as your iMac does not have a separate video card. Good job your iMac is under warranty, as the board replacement would be expensive. I work with Mac's, but as yet haven't seen the fault you describe on yours or a similar model, only a few no video faults, black screens. Freezing couldn't be caused by the LCD.

This sounds identical to the problem i had with my iMac a few years ago. It was an 2008 model but stil. It started with the screen freezing from time to time and after a few weeks the gpu died . Id bring it back while you can