SQL INSERT Query takes 2 seconds and sometimes much longer to execute I know the question's title is bad, But I have  an hard time figure it out my self...
sometimes this query runs in 2-3 seconds and sometimes it's takes even 30 seconds

I have a table named ranking_temp_free:


I have one Index except the primary key:



I have no triggers or partitions on this table..

I'm using this Query:

INSERT IGNORE INTO ranking_temp_free 
(countryID, categoryID, ranking, popID, app_id, check_time) 
VALUES (/*Here comes 50,000 rows of Data*/);


And as I wrote before.. The time that takes the query to finish is not fixed as I wish to be...

Logs:

THERE ARE MORE LOGS BEFORE..

07-02-2013 00:24:37 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:24:54 Inserted: 50000 rows
QUERY TIME : 17 SECONDS

07-02-2013 00:25:05 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:25:41 Inserted: 50000 rows
QUERY TIME : 36 SECONDS

07-02-2013 00:26:02 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:26:07 Inserted: 50000 rows
QUERY TIME : 5 SECONDS

07-02-2013 00:26:28 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:26:35 Inserted: 50000 rows
QUERY TIME : 7 SECONDS

07-02-2013 00:26:46 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:26:52 Inserted: 50000 rows
QUERY TIME : 6 SECONDS

07-02-2013 00:27:03 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:27:17 Inserted: 50000 rows
QUERY TIME : 14 SECONDS

07-02-2013 00:28:19 Sending SQL: insert ignore into ranking_temp_free
07-02-2013 00:28:39 Inserted: 50000 rows
QUERY TIME : 20 SECONDS

AND SO ON.........


It's a simple INSERT so why the time is different?.

Just to be clear: THERE ARE NO ANY OTHER PROCESSES ON THE SQL SERVER!


EXAMPLE FOR THE TABLE DATA:
We found the solution to the slow random query execution time:

I added this line: SET TRANSACTION ISOLATION LEVEL READ COMMITTED right after the DB connect.

we are using PHP, So it's looks like this:

mysql_select_db($db, $link);
mysql_query("SET TRANSACTION ISOLATION LEVEL READ COMMITTED");


All our problem solved with this magic line.

Now our queries run in 40 minutes and not in 2.5 hours as it use too..

thanks all for your help.