I had a similar issue on a 7400 when I upgraded to 4.14k. System would restart once a week at around the same time every week. I originally thought it was an external issue (power) but proved the power was ok.

I thought something was corrupted during the upgrade or when I had moved some cards around (an 16SLIMW would lose dial tone in 1 slot only, main cabinet, swapped cards around and the issue stayed with the slot).

I did a backup using OIT.
Defaulted system by setting MP40 Memory Backup switch to Off.
Rebooted with MP40 Memory Backup switch On.
Did another backup to a temporary file with the defaulted data using OIT.
Restored the original database using OIT.
Did another reboot.

I was going to then program up the temporary database to match the existing but since I've done the previous operation, the system has been stable AND the 16SLIMW in the bad slot still has dial tone.

This was back in August. I've since upgraded to 4.22 with no issues.