Scheduler Job overrunning intermittently

Hello Experts,

   I have a scheduler Job(which runs a Procedure and this procedure has one INSERT statement) that is expected to run 3-4 mins, most of the days it runs as expected, Hoever suddenly it over runs for 4-5 Hrs and never completes successfully, Even I stop the Job and rerun it it will keep running. During those days I have to stop relying on running job rather run the INSERT statement as is. It will execute in the expected 3-4 Mins.

   I couldnt see anything any abnormal in the AWR reports, nor any locks.

   The count of Job_queue_processes in the database is 10 and at this time of the morning only this Job runs in the database, not sure if these processes are causing any problem.