2013 iMac Screen On & Off on a Normal Boot

My wife's 2013 iMac's screen is going dark for a few seconds, then goes on for half a second (figuratively, I haven't timed it). It's on for too short of a time to do anything.

I added another monitor to see if that would help.

The added monitor worked fine, but was the secondary monitor.

I have booted to Recovery Mode, and the screen stays on constantly, but 'recovery' seems to be stuck at 24 minutes!

I have also rebooted in Safety Mode, which was much better.

Once it finished booting in Safety Mode, both monitors flickered.

Then, at times, the iMac monitor would flicker just in certain areas (NOT the whole screen).

I switched the primary screen to the secondary monitor & rebooted.

Now the iMac monitor still 'flickers' (off/on), but the secondary monitor is working as it should.

UPDATE........

My wife went to cover the iMac monitor because of the on/off issue, and as soon as she covered it and was ready to put on some tape, the monitor stayed ON!!!

I'll keep the the setup as-is for now, just in case the issue re-appears, but this is ODD!!