Imac Keeps Shutting Down during use and needs to be reset

[lifeline]

Just stated to have this problem with my 2013ish iMac 27in. During use it just shuts right off. Sometimes ill get a day out of using it then it shuts down. After it shuts down, I have to unplug it to restart it. Sometimes after restart I might be a few minutes of use before it happens again.
Apple ran a bunch of test over the phone and also reinstalled operating system and still didn't work.
I then brought it into the apple store, they had it for a week and could not duplicate the problem. After getting it back I used it for a few days and all good then it happened again, just my luck. Nothing extra is plugged into the computer, any suggestions would be greatly appreciated. Last year the hard drive failed on this computer and I had to replace that as well.

 

[hfg]

Do you have any kind of power conditioner on your AC line going to the iMac such as a UPS (uninterruptible power supply) or Surge Suppressor? You might be experiencing power line issues which are causing problems in your iMac. You also might check your line voltage where you plug the iMac in for power to verify it is not too low ... or high.

 

[lifeline]

Thanks for the reply. I had it plugged into my UPS. But when having this problem i just plugged the computer into a different wall outlet without the UPS and still had the same problem.