Database crawling because same multiple queries having different execution plans

Hi,
we are facing database running very slow and noticed that Cognos is submitting
reports with same queries multiple times with different execution plans.
Does anybody have an idea what is causing the crawling and how to eliminate
different execution plans for same queries? Noticed also that same queries
sometimes using index and sometimes not using index (full table scan).

Database version: 10.1.0.4.0

Thank you