iMac 27" Random Power Shutdown

I am having major issues with my iMac (Retina 5K 27" Mid 2015) i5 running Sierra. The mac will randomly drop all power, and you need to pull the power cable to restart the unit. Had it to my local iStore, and did the following:

1) Ran diagnostics - All clear, and no faults

2) Reinstalled Sierra (Ver 10.12.3) - Still randomly shuts down

3) Uninstalled every application to go back to vanilla iMac - Still randomly shuts down

Does anyone have a remedy to this issue, as it seems that Apple don't know what to do next?

Appreciate any help!