Issue with proc - erratic behaviour

Question

I have 2 tables A and B. A and B are populated by 2 large queries. After populating A and B another procedure runs on A and B and inserts data after processing into another table C.
The procedure consists of multiple cursors. A, B and C are truncated before data load.
The issue is as follows:
The procedure runs in around 1-2 minutes. But, at times it slows down considerably taking approx.20-30 minutes to run. If table A and B are truncated, dropped and recreated then the same procedure runs again in 1-2 minutes. What could be the issue?
No other batch is running at the same time. Has anybody encountered this situation before?

Answer 1

So, to re-cap.
Situation 1. Tables are truncated. Process run. Performance problems encountered.
Situation 2. Tables are dropped and re-created. Process run. No performance issues.
Did I understand it correctly?
The only difference is the fact that the tables are re-created and not truncated. A couple of things comes to mind that is different.
With the tables being re-created, the statistics for that table is now reset. I'm not sure if this is too the case for a truncate.
A truncate does not release the space allocated to the table, back to Oracle as free space, when the REUSE STORAGE clause is used. In contrast, a DROP TABLE followed by a CREATE TABLE will have a different space allocation.
A DROP TABLE also of course removes triggers and index objects for that table. A TRUNCATE TABLE does not.
Somewhere in these list of differences (am sure there should be a few more that is applicable), the cause and reason for that performance discrepancy should be found.