Reporting Service- Connection Problems to the oracle data-mart

I have a reporting service that is connected to an Oracle data-mart. Sometimes the connection gives the following error:
An error has occurred during report processing. (rsProcessingAborted)
Cannot create a connection to data source 'CA'. (rsErrorOpeningConnection)
ORA-12640: Authentication adapter initialization failed
The connection to the oracle data mart:
- Data source Type: Oracle
- Connection string: Data Source=mamaroso;User ID=w6admin;Unicode=True
- Credintials stored securly in the report server(Populated with a valid username and password)
I use the same connection in the Visual Studio 2005 SSRS project and it allways works fine(On the same machine).

After 'playing' with the connection configuration and returning to the setting described above, the connection works fine- But this is no solution! I must understand the problem and know exactly what steps to take to fix it.

Thanks,
Rami