Sql Query Timeout, Expensive Cast Operation?

I need some tips to boost the performance on the following query.
The problem is that it times out once in a while, and then again runs
normally in most cases.
The clue is to compare a textual value stored as an image data type, but I
don't know if the cast operation could be done in another way.
Any tips?

SELECT DISTINCT pk_product FROM tbl_product P
JOIN tbl_product_content PC ON P.pk_product = PC.fk_product
JOIN tbl_content C ON PC.fk_content = C.pk_content
JOIN tbl_content_rel REL ON C.pk_content = REL.fk_content
JOIN tbl_filegroup FG ON REL.fk_filegroup = FG.pk_filegroup
JOIN tbl_filegroup_file FF ON fg.pk_filegroup = FF.fk_filegroup
JOIN tbl_file F ON ff.fk_file = F.pk_file
WHERE
P.fk_product_type = 45 AND
CAST(CAST(f.data AS VARBINARY) AS NVARCHAR(20)) = '1234'


/ted