Why does iMac shut down suddenly even just after starting

Will run for several hours if just on desktop with NOTHING (e.g. menu, options, Finder) is opened. I.e. just sitting idle.

As soon as I click on ANYTHING, it will shut down just as if power cable is disconnected, any time from 30 secs to 4 minutes later.

IMac is 11 months old. In last month or two will not run for any time unless it is idle !

Has been back to retail store they found no errors / faults with any H/ware. Reported all normal.

I have ;

- not added any new software for over 8 months (installed Creative Cloud)

- replaced batteries in bluetooth keyboard & mouse

- disconnected Google Drive

- let system update some software such as Safari and some security s/ware

- NOT updated original from OS X Mavericks

This runs my Pro Tools 11.3 install and has done since new, without problems but now is just taking up space on my desk - it is virtually useless !!

I would REALLY appreciate any assistance to resolve this and get back to a usable machine.

Thanks in anticipation !