why stored procedures query sometime taking little time to execute

ALTER PROCEDURE [dbo].[UpdateSatckWithBookID] 

@movie_ID int,
@screen_ID tinyint,
@class_ID  tinyint,
@movie_ShowDate smalldatetime,
@show_ID tinyint,
@row_No smallint,
@col_No smallint,
@is_Available bit,
@is_Counter char(10),
@book_DateTime smalldatetime,
@book_IPAddress varchar(15)

as
set nocount on
set transaction isolation level serializable
begin transaction


Update movie_Show_Stack set is_Available=@is_Available,is_Counter=@is_Counter,book_DateTime=convert(smalldatetime,@book_DateTime,103),book_IPAddress=@book_IPAddress  where row_no=@row_No and col_No = @col_No and screen_id=@screen_ID  and class_id=@class_ID  and Show_ID=@show_ID and Movie_ID=@movie_ID and movie_ShowDate =convert(smalldatetime,@movie_ShowDate,103)


if(@@error<>0)
begin
 rollback transaction
 return -1
end
else
begin
 commit transaction
 return 1
end