My MacBook is experiencing Random Hard Freezes when unattended

Appreciate any guidance on solving this mystery. I have been experiencing random hard freezes on my Macbook for a few weeks now. For what it's worth, it has never happened while I was actively using the machine, but if I leave it idle and step out of the room, I'll return to a frozen state, i.e. mouse doesn't move, keyboard unresponsive. It seems truly frozen because if I close the lid to put it to sleep in this state, it never goes to sleep (well, not within a reasonable span of time, I haven't left it in that state). I have to hard reset by holding down the power button.

I've run Apple's extended hardware test and it finds no issues. I've run another memory test that also shows no problems, and I've even tried swapping out my Macbook's original memory and experienced the same issue. SMART utility said my hard drive had errors and was failing so I hoped it was that, but I used super duper to copy everything over to a new drive and the issue persists.

I've resisted a totally clean install because I hate the idea of having to get all my software installed again, etc, but I'm fast coming to trying that if there's a chance it will resolve this, i.e. that it's not a hardware issue.

The only crash report I found did not coincide with a freeze but I removed the software (Logitech Control Center) that crashed to no avail. If there are any other details I can provide that would help narrow this down, please let me know!