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After the upgrade the system works fine if I am working at it but if I leave it
for 10 minutes or more it will reboot itself or lock hard (I need to power off
and on again). Before the upgrade the system ran solidly.

I saw something similar a couple of years ago in 15.1 but the upgrade from 15.1
to 15.2 fixed it. This upgrade has reverted to the system rebooting itself or
locking hard.

Reproducible: Always

Steps to Reproduce:
1. upgrade Intel NUC from 15.2 to 15.3
2. leave the system inactive for 10 minutes or more (or until the screen blanks
?)
3. 
Actual Results:  
The system has rebooted or is locked hard.

Expected Results:  
The system should keep running.

Window manager: icewm
lsmod | grep video    # video                  53248  1 i915
Intel NUC NUC7i7DNHE + Samsung 1TB 970 PRO V-NAND M.2 + Ballistix Sport LT 32GB
Kit.
I have captured /etc from 15.2 and 15.3 but cannot see anything obvious in the
diff
I am happy to help provide any assistance from my end. Just tell me where to
look.