TNS-12516, TNS-12519, ORA-12520 Intermittent or fleeting

These are listener log or service handler errors.
These errors can occur for a variety of reasons. 
When they're thrown consistently for all remote connections, it's likely a configuration issue.

Look first to LOCAL_LISTENER and REMOTE_LISTENER settings in RAC.

When they occur and then "self correct" or clear up with no dba intervention, look to PROCESSES first.

Log in to the offending instance and issue:

select * from v$resource_limit;

or RAC:

select * from gv$resource_limit;

Examine the PROCESSES values closely. 
If MAX_UTILIZATION is approaching the LIMIT_VALUE, the problem is clear.

The setting is inadequate and PMON has blocked the handler at the listener so