After unremarkable operation for several weeks on Raspbian Stretch, in February 2018, I ran my first apt-get update and upgrade, which resulted in flakiness in the display at startup, in wireless connection, and MagicMirror operations. Ugh! Several weeks of fruitless troubleshooting and reinstalls ensued. I grudgingly went back to the base Raspbian Stretch installation, but strangely MagicMirror itself had now unstable. I largely gave up in the hopes that updating at a later time would solve the problem.
Then, I stumbled across a post on another forum that pointed to a wireless firmware driver that gets broken during update. It’s possible to tell update to skip a program or driver with apt-mark. Running this before update and upgrade appears to have solved all problems:
sudo apt-mark hold firmware-brcm80211
apt-get update and upgrade ran without incident. There were no aberrations on the display at startup, the wireless connection accepted configuration normally and ran without incident, and MagicMirror installed and configured without incident. Essentially, a reprise of my positive experience when I first put together my MagicMirror.
My mirror has been running for several hours as configured without incident, and with no hint of a problem. If this should change in the next few days, I’ll add to this post, but right now I’m declaring this noisome problem solved. Hope this helps other fellow sufferers.