Explain Plan irregularity

While tuning a pl/sql procedure, I've noticed that my execution plan will change after I hard code the dates in a between condition for a insert into select statement. The between is meant to use passed in variables to run for different periods. In fact, if I hard code the dates the query will finish in 30 secs while passed in date variables will run in about 15 minutes.

i.e.WHERE inv_date between '01-FEB-2002' and '01-MAR-2002'; if.WHERE inv_date between p_start_date and p_end_date;

Does anyone know why this is? Is there anything I can do about it?

Thanks

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