NLS_TIMESTAMP_FORMAT changing automatically and randomlly
First change the parameter at the system level:

alter system set nls_timestamp_format = 'dd/mm/yyyy' scope='both';


This will also update the spfile so even if you bounce the db you'll keep your definition. 

Second, check for any specific definitions in your client.
If you're using windows go to


the registry - check if you have the nls_timestamp_format set differently.
the glogin.sql file - this is a file that executes every time you log into sqlplus. there might be an alter session... there that changes the system setting.