What's the problem with my thread ?

Hi,
Here is the code of a small program that calls a simple
thread to do nothing more then update a label.
The problem is that when I call the thread more then once
it is no working correctly. I mean that the program is
writing the value of the string in the upper left corner of
my screen. If I launch even more threads, then I get the
error : "canvas does not allow drawing"
I thought that I was doing things right by using a proc passed to the main thread with syncronize.
Here comes the code, let my know what you think about it
please. If you think the question deserves more points, let me know too. Thanks in advance for your help.
If you want me to send you the project in a zip file,
give me an email...

---------the thread unit
unit Unit2;

interface

uses
     Classes;

type
     TSnoopDir = class(TThread)
     private
          { Private declarations }
          procedure DoVisualFeedBack;
     protected
          procedure Execute; override;
          procedure VisualFeedBack;
          procedure test;
     public
          constructor Create;
     end;

implementation

uses unit1, sysutils;

{ SnoopDir }

constructor TSnoopDir.Create;
begin
     FreeOnTerminate := True;
     inherited Create(False);
end;

procedure TSnoopDir.DoVisualFeedBack;
var
   a,b :integer;
begin
     a:=random(1000);
     b:=random(1000);
     Form1.label1.Caption := IntToStr(a);
     Form1.label2.Caption := IntToStr(b);
end;

procedure TSnoopDir.VisualFeedBack();
begin
     Synchronize(DoVisualFeedBack);
end;

procedure TSnoopDir.test;
var i:integer;
begin
     for i := 0 to 1000 do
     begin
          DoVisualFeedBack;
     end;
end;

procedure TSnoopDir.Execute;
begin
     test;
end;

end.
---------------------


------the project--------
program Project1;

uses
  Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas';

{$R *.RES}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.
---------------------
-----the form : only a button and two labels
unit Unit1;

interface

uses
     Windows, Messages, SysUtils, Classes, Graphics,
     Controls, Forms, Dialogs, StdCtrls;

type
     TForm1 = class(TForm)
          Button1: TButton;
    Label1: TLabel;
    Label2: TLabel;
          procedure Button1Click(Sender: TObject);
     private
     public
          { Public declarations }
     end;

var
     Form1: TForm1;

implementation

uses unit2;

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
begin
     TSnoopDir.Create;
end;

end.
-------------
-------the dfm
object Form1: TForm1
  Left = 499
  Top = 87
  Width = 159
  Height = 117
  Caption = 'Form1'
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Sans Serif'
  Font.Style = []
  PixelsPerInch = 96
  TextHeight = 13
  object Label1: TLabel
    Left = 16
    Top = 56
    Width = 32
    Height = 13
    Caption = 'Label1'
  end
  object Label2: TLabel
    Left = 88
    Top = 56
    Width = 32
    Height = 13
    Caption = 'Label2'
  end
  object Button1: TButton
    Left = 40
    Top = 16
    Width = 75
    Height = 25
    Caption = 'start'
    TabOrder = 0
    OnClick = Button1Click
  end
end
----------------