Help Second Posting Due to No Response!

My iMac shuts down unexpectedly, often, when it does, it has to be unplugged and plug back in or startup button won't work.

I have ran Mac Diagnostics, no issues were found. Replaced surge protector and power outlet.

Taken it to Apple store, they did not find any issues, yet this problem persists.

Model Name: iMac

Model Identifier: iMac14,2

Processor Name: Intel Core i5

Processor Speed: 3.4 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 32 GB

Boot ROM Version: IM142.0118.B13

SMC Version (system): 2.15f7