Stored Procedure is taking longer than raw SQL

Hey Experts,

I am seeking help for a very strange issue I am having with a stored procedure and it's execution time.

I have a store procedure that when run on its own (as a set of queries) runs in about 3-5 minutes.  However, when I execute it as a stored procedure it takes over 30 minutes to run.  This stored procedure is pretty long and hits several views and tables, but no other stored procedures.

Here is the weird part, if I declare new variable that are the same as the ones being passed to the procedure, then assign the new variables to the values of the parameters being passed, it runs in 3-5 minutes again.  I know this last bit is a little confusing, so below is an example.

This is a SQL server running MS SQL 2005.  Also, I have already tried recompiling the stored procedure, and that hasn't helped the problem.  

Example:

Raw Queries (takes 3-5 minutes to run)

DECLARE @VariableA VARCHAR(50)
DECLARE @VariableB VARCHAR(50)
DECLARE @VariableC VARCHAR(50)

SET @VariableA = 'ABC'
SET @VariableB = 'DEF'
SET @VariableC = 'GHI'

SELECT * FROM [FirstSelect] WHERE ColumnA = @VariableA
SELECT * FROM [SecondSelect] WHERE ColumnB = @VariableB
SELECT * FROM [LastSelect] WHERE ColumnC = @VariableC

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As Stored Procedure (Takes 30+ minutes to run as a stored procedure)

CREATE PROCEDURE myStoredProcedure
	@VariableA VARCHAR(50) = '0',    
	@VariableB VARCHAR(50) = '0',    
	@VariableC VARCHAR(50) = '0'
AS
	SELECT * FROM [FirstSelect] WHERE ColumnA = @VariableA
	SELECT * FROM [SecondSelect] WHERE ColumnB = @VariableB
	SELECT * FROM [LastSelect] WHERE ColumnC = @VariableC
GO

EXEC myStoredProcedure 
	@VariableA = 'ABC', 
	@VariableB = 'DEF', 
	@VariableC = 'GHI'

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Very strange way that makes the stored procedure run faster (3-6 minutes)

CREATE PROCEDURE myStoredProcedure
	@VariableA VARCHAR(50) = '0',    
	@VariableB VARCHAR(50) = '0',    
	@VariableC VARCHAR(50) = '0'
AS
	DECLARE @VariableA1 VARCHAR(50)
	DECLARE @VariableB1 VARCHAR(50)
	DECLARE @VariableC1 VARCHAR(50)

	SET @VariableA1 = @VariableA
	SET @VariableB1 = @VariableB
	SET @VariableC1 = @VariableC

	SELECT * FROM [FirstSelect] WHERE ColumnA = @VariableA1
	SELECT * FROM [SecondSelect] WHERE ColumnB = @VariableB1
	SELECT * FROM [LastSelect] WHERE ColumnC = @VariableC1
GO
EXEC myStoredProcedure 
	@VariableA = 'ABC', 
	@VariableB = 'DEF', 
	@VariableC = 'GHI'
GO

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