My iMac (Retina 5K, 27-inch, 2017) continually restarts itself, followed by the message, "Your computer restarted because of a problem."

I've continually encountered a problem with my 27" iMac restarting, followed by the above message (indicated in the subject line). Strangely, if I remain working on the computer--like now--the issue doesn't arise; it's only if I step away for a few minutes it occurs. Without fail. A few months ago, I even spoke with an Apple tech support person, who guided me through basic troubleshooting steps (involving resetting some System settings; can't specifically recall what they were) The tech also had me update to the latest OS. That, however, failed to fix the problem. Since I'm reluctant to pack up my iMac for a trip to the Apple Store, I've basically "lived with" the problem. (I just shut down my Mac after each use to prevent it from constantly restarting!) This evening, after the machine alerted me to the availability of a new OS upgrade, I manually performed the action (updating to OS Catalina ver. 10.15.3). Before the upgrade was complete, I walked away for dinner. When I returned and signed on, I got the same dreaded error message AGAIN. Therefore, can any knowledgable, technically astute person offer a helpful suggestion ... before I finally relent and pack this thing off to the Genius Bar?? Thank You!