iMac Unexpectedly Powers Down

Getting really frustrated at my less than 2 year old iMac that for seemingly no reason at all, powers down all the way to turning off. At that point, the power switch does not turn it back on, either. I have to literally unplug the wall plug, then plug it back into that socket, at which time the unit will power up and do a normal boot.

This happened about 6 weeks ago, so I took it into the local Apple Store for analysis and repair. They put it on their bench and started it up and it never went down for 5 days. They ran some sort of program that taxed the system also during that time. I brought it home, plugged it back in but to a different wall outlet. The unit ran fine for the last 6 weeks and again, this morning, just powered down right in the middle of a browser search.

I’m beginning to think that It might have to do with upgrading to Sierra. I hadn’t had any problems at all until some time after upgrading to Sierra. I have since gone to High Sierra. Sometimes the unit will stay up for an hour or so, sometimes only a few minutes. Sometimes less that 2 minutes. It’s maddening.

Has anyone experieneed this? If so I would appreciate any insight.

Thank you!