iMac Random shutdown

I'd be investigating what is different at home that at the shop...

Could your AC power have a ground loop or worse?

Any cables plugged nat home that were not at store? ie. Ethernet, printer, other accessories?

We need to see what all is running, a report from this will not display any personal info...

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

vogrecords wrote:

The thing is that I have to pull out the power plug, wait 15 seconds and then replug it and it comes back on. The longer I keep it unplugged, the longer and better the computer works.

That's a sign of a failing power supply. I went thru the same think with a 2014 iMac. First thing to do is make sure you have a full and current backup of all your user data.

If you take it into Apple for repair the first thing they'll do is erase the drive and install a pristine system to verify it's a hardware issue and not a software one. So make sure you have that backup.