Oracle Query execution finishes first time but hangs up from second run
This is quite typical on 11.2 because of a feature called "cardinality feedback". Check the execution plan of the slow runs, and if you see "cardinality feedback" in the Notes section of the execution plan, try setting the below parameter: alter session set "_optimizer_use_feedback"=false;, and run your tests again.

Some bugs related to this:

Bug 12557401 - Suboptimal plan on 2nd execution caused by cardinality feedback (Doc ID 12557401.8)

Bug 16837274 - Cardinality feedback produces poor subsequent plan (Doc ID 16837274.8)

Bug 8608703 - SubOptimal Execution Plan created by Cardinality Feedback (Doc ID 8608703.8)

Bug 8521689 - SubOptimal execution plan on second execution of GROUP BY query (Doc ID 8521689.8)