urgent - sql table does not work any more as i tshould

I get 1500 rows in 00:00:00 seconds when I select top 1500 rows like this

select top 1500 * from MyTable

If I search for 1540 top rows

select top 1540 * from MyTable it runs for 10 seconds before I stop it.
It returns 1500 rows right away but after that no more.

I have been using this table for a long time and never had a problem with it until now.

Here is the table

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

CREATE TABLE [dbo].[MyTable](
      [MyId] [int] IDENTITY(1,1) NOT NULL,
      [MyText] [varchar](max) NULL,
      [PId] [int] NULL,
      [YearId] [int] NULL,
      [DateCreated] [datetime] NULL,
      [CreatedBy] [int] NULL,
      [Shortname] [varchar](10) NULL,
 CONSTRAINT [PK_s_Class] PRIMARY KEY CLUSTERED
(
      [MyId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

SET ANSI_PADDING OFF
GO

ALTER TABLE [dbo].[MyTable]  WITH CHECK ADD  CONSTRAINT [FK_MyTable_s_Payer] FOREIGN KEY([PId])
REFERENCES [dbo].[s_Payer] ([PId])
GO

ALTER TABLE [dbo].[MyTable] CHECK CONSTRAINT [FK_MyTable_s_Payer]
GO

EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'Some text' , @level0type=N'SCHEMA',@level0name=N'dbo', @level1type=N'TABLE',@level1name=N'MyTable'
GO

I am not sure why but now it works.
Maybe I was running some query at the table. I closed all sql windows and then it worked:-)

which column was added?

have you tried to update statistics for that table?

is there any ORDER BY involved in your TOP 1500 or 1540 queries?

& just curious: is it really called [MyTable]?