HI again
I just tried running Universe Sandox again, and after 4 hours my system slowed right up again and I had to do press Alt-print-K to log out. Moreover, when I connect the HDMI to my Optama Projector, I still get the same video tearing and the only solution is to boot into my Debian stretch testing partition.
I also ran Universe Sandbox for almost five hours in Debian stretch testing, and again, unlike stable, it ran with no problems.