Slow INSERTs On A Table

Hello all. I've got a problem with really slow INSERTs on one (and only one) of the tables in a database. For example, using SQL Management Studio, it takes 4 minutes and 48 seconds to insert 25 rows. There are only about 8 columns in the table and only about 1500 records. All the other tables in the database are very fast for inserts.

Another odd thing uniquely associated with INSERTs on this table: prior to inserting the 25 new rows of data, SQL Management Studio tells me that it inserted 463 rows of data which I know did not happen. Here's the INSERT statement:

INSERT INTO FieldOps(StudySiteID
, QA_StructureID
, Notes
, PersonID)
SELECT DISTINCT StudySiteKey
, QA_StructureKey
, SampleComments1
, '25'
FROM ScriptOutput_Nitrate
WHERE (ScriptOutput_Nitrate.StudySiteKey IS NOT NULL)

and SQL Management Studio (eventually) says:
(463 row(s) affected)
(463 row(s) affected)

(25 row(s) affected)

The table has an index on the primary key (INT data type with auto increment). I tried running the following code to fix things but it made no difference:

USE [master]
GO
ALTER DATABASE [FieldData] SET SINGLE_USER WITH ROLLBACK IMMEDIATE
GO

use FieldData
GO
DBCC CHECKTABLE ('FieldOps', REPAIR_REBUILD) With ALL_ERRORMSGS
GO

USE [master]
GO
ALTER DATABASE [FieldData] SET MULTI_USER WITH ROLLBACK IMMEDIATE
GO

I'm guessing that the problem might be related to the index (??). I don't know... Does anyone here have a suggestion as to what I should do to fix this problem.