I’ve owned and deployed a lot of pi, every model, and in my experience when I have similar instability as you described its related to the sdcard. Either the sdcard itself or the tray soldered to the pi. I had one pi that would corrupt the sdcard without fail after 2 months and I played with bending the sdcard metal tray inward a little to help press the card better into the contacts and the problem went away. Try fiddling with the sdcard holder or different sdcards.
Shit I thought I was so damn novel. Blast you.