Last week I was able to test all 6 sticks of RAM. And one of them always caused the RAM test to fail (very quickly).
With this one stick of RAM removed, my PC has been running Zorin 18 for about 5 days and the hard crashed have stopped. My PC feels very stable now.
I made a bad assumption that my hardware was healthy because I never experienced issues (crashing) running Windows 10.
Does anyone have any ideas why Windows was able to tolerate this bad stick of RAM while it was a problem for Linux?
Even now, if I were to put my SSD with my Windows 10 install back into my PC (with all RAM, including the bad stick) Window will run and I won't see these hard crashes.
Regardless, I'm down to 20 GB RAM (from 24 GB) and I'm very happy to be on Zorin!
Yes, DDR3 RAM is not expensive, I just need to find a 4 GB stick of:
Kingston Hyperx 9905403-437 .a01lf
or
Kingston Hyperx 9905403-533 .a00lf
To match what I already have.
Thank you all for your replies and help. I really do appreciate it.