Full text query over multiple tables

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SQL-Server-2005

SQL

SQL-Server

, +

I have two tables, one called parts, and one called customer_parts. The first is our catalogue of parts available, and customer_parts is a matrix of each of our customers' part references. The basic schemas are...

parts:

part_number | description | [other fields]...
--------------------------------------------------
            |             |

customer_parts:

customer_id | part_number | customer_part_number
--------------------------------------------------
            |             |

I have a full text catalogue that contains both tables, and the following fields:
parts.part_number
parts.description
customer_parts.customer_part_number

Below is a cutdown version of a query I run, and it runs extremely fast. So far so good...
@CustomerID and @SearchTerm are inputs into the stored procedure

DECLARE
	@FTS_CONTAINS varchar(200),
	@FTS_FULLTEXT varchar(200)

-- Generate full text queries
SELECT @FTS_CONTAINS = '"' + ISNULL(@SearchTerm, '') + '"'
IF IsNull(@SearchTerm, '') = ''
	BEGIN
		SELECT @FTS_FULLTEXT = '""'
	END
ELSE
	BEGIN
		SELECT @FTS_FULLTEXT = @SearchTerm
	END

-- Lookup matching parts
SELECT *
FROM parts p
	LEFT JOIN customer_parts i ON p.part_number = i.part_number AND i.customer_id = IsNull(@CustomerID, -1)
WHERE (IsNull(@SearchTerm, '') = ''
	OR CONTAINS(p.part_number, @FTS_CONTAINS)
	OR FREETEXT(p.description, @FTS_FULLTEXT)
	)

However, when I add the following line into the OR conditions, my query suddenly takes forever to execute. Take it back out, and it's instantaneous.

OR CONTAINS(i.customer_part_number, @FTS_CONTAINS)

I'm afraid I'm pretty new to full-text querying, so I'm a bit stuck. Any help would be appreciated.

Thanks