HA Working on Pi3B only for short stints

My HA works off a Raspberry PiB for 2-3 months at a time.

I have gone from micro SD to a external usb memory stick but the same thing happens.

The HA OS becomes inaccessable from the App, I then have to pull the power to reboot it and it never comes back online.

I start the complete install from scratch and restore from last backup to retain my automations and settings.

Whether I run updates / none the OS becomes unstable. How does one remedy this?