Recurring black flashing screen problem on imac 5k 27

Hi, I've been having a recurring problem and wondering what should be done about it or if anybody else has heard anything like this. By recurring, i mean that I've had this problem on 3 imacs now- i've been trying to upgrade to an imac for the past two months and have made three returns so far and am having this problem on this fourth one now (the first imac didn't have this problem, but would often crash unexpectedly. the second imac crashed often too and has this problem. this imac and the last one seem to have only had this problem).

The problem is:
Occasionally (about 1-2 times per hour), the screen of my imac will turn black for a 2-5 seconds, and then it will come back to normal. This seems to happen regardless of what i'm doing on the computer (ie could be when just browsing internet, writing a doc, or video editing). The computer operates fine otherwise- its sometimes annoying to have the screen go black while i'm working, but i'm mostly concerned that it could be a sign of a problem that could get worse over time.

I don't know if this is normal or i've just had bad luck or if something on my computer is causing it, but its never happened with two macbook pros that i've used daily before and apple phone support has said several times they don't know what it is. I'm tired of making returns of this device but dont want to get stuck with a bad computer- wondering if i should just keep it and risk it, return and get another imac, take it to the genius bar, or try getting a mac pro instead.

Is it normal for these imacs to have problems? Any advice?

The computer is:
Late 2015 iMac Retina 5k 27 inch running latest OSx
with 2tb fusion drive, 8gb ram, the upgrade apple offers for the video card, and the upgrade apple offers with the processor. Running only apple and popular apps (nothing that makes me think it could be causing a problem).

Thanks for your help,
Dan

That is absolutely NOT normal. Those iMacs should have been replaced immediately. Get them to an Apple store ASAP to have a hardware diagnostic done. You definitely have a serious hardware problem that will only get much worse, until the displays are totally gone for good.

With that many Macs, you should consider a business account with your local Apple retail store. You get faster service, slight discounts, etc.

It is very strange that you have the same problem with 3 different iMacs. I think you have problem with power fluctuations in your home / office that are causing the iMacs to behave strange. Try using a different power socket in a different room or take computer to a friend's house for testing. Also you could buy a UPS power supply that stabilize the power fluctuations and delivers clean power to your iMac.

Do you ever see anomalies in other appliances in that building?

Thanks for the replies. I have been using a UPS (and haven't had problems with any other devices with it), though i will try plugging it into a different surge protector to see if that makes a difference. I have tried it in two different office spaces.

Thanks for the recommendation about the business account too