Query Duration

During a migration of a database, a remote export/import procedure (through database-link) fails intermittently. Some tables are skipped during export, with the familiair ‘ORA-0155’ error. Snapshot too old. Procedure hasn’t changed. What’s going on, besides the obvious reasons for an ORA-0155. Database version changed from 10.2.0.4 to 11.2.0.4.