Imac Shuts down randomly
Where to begin. have a 2 yr old iMac 27" 3.6 Ghz - 8 core Intel i9/ Memory 64 GB. Updated to Big Sur. During installation it went to black and did not come back on until it was unplugged and restarted. Afterwards everytime we went to random file folders it would just shutdown. And only way to restart was to unplug from wall first. Brought it to Apple Store to diagnose and after a week it came back with no issues found. So we completely reinstalled everything fresh - did not use migration / time machine in case it kept copying over the issue (corrupt file or something). Worked fine for 2 weeks and now it has crashed 6 times today. We're just holding the space bar to preview a file - so not a heavy operational lift at all and then crash! And then again on restart and just logging in it crashed.
Not sure what to do now! Any suggestions?
iMac 27″, macOS 10.13