熟悉GDI的朋友帮我看看代码哪儿错了

程序开始时还可以，没有错误，但当执行一段时间（不会很长）后出现错误，请帮我调试
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls;type
  TForm1 = class(TForm)
    Timer1: TTimer;
    PaintBox1: TPaintBox;
    PaintBox2: TPaintBox;
    procedure FormCreate(Sender: TObject);
    procedure Timer1Timer(Sender: TObject);
    procedure PaintBox1Paint(Sender: TObject);
    procedure PaintBox2Paint(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;
  i,j,k:integer;implementation{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);
begin
  i:=201;
  j:=398;
  k:=0;
end;procedure TForm1.Timer1Timer(Sender: TObject);
begin
  //Interval=1
  i:=i+1;
  j:=j-1;
  k:=k+1;
  Form1.Caption:=IntToStr(k);
  PaintBox1.Refresh;
  PaintBox2.Refresh;
  if i=398 then i:=200+random(198);
  if j=201 then j:=398-random(197);
end;procedure TForm1.PaintBox1Paint(Sender: TObject);
var
  rgn,rgnb,rgnt,rgn1,rgn2:THandle;
  hBrush:THandle;
  Rect1,Rect2:TRect;
begin
  try
  PaintBox2.CleanupInstance;
  Rect1.Left:=100;
  Rect1.Top:=200;
  Rect1.Right:=300;
  Rect1.Bottom:=400;
  rgnb:=CreateEllipticRgnIndirect(Rect1);
  rgnt:=CreateEllipticRgnIndirect(Rect1);
  rgn1:=CreateEllipticRgnIndirect(Rect1);
  rgn2:=CreateRectRgn(100,200,300,i);
  CombineRgn(rgnb,rgn1,rgn2,RGN_DIFF);
  CombineRgn(rgnt,rgn1,rgnb,RGN_DIFF);
  CombineRgn(rgn,rgnb,rgnt,RGN_AND);
  //{
  PaintBox1.Canvas.Brush.Color:=clRed;
  if not FillRgn(PaintBox1.Canvas.Handle,rgnb,PaintBox1.Canvas.Brush.Handle) then ShowMessage('rgnb Failed');
  PaintBox1.Canvas.Brush.Color:=clBlue;
  if not FillRgn(PaintBox1.Canvas.Handle,rgnt,PaintBox1.Canvas.Brush.Handle) then ShowMessage('rgnt Failed');
  //}
end;procedure TForm1.PaintBox2Paint(Sender: TObject);
var
  rgn,rgnb,rgnt,rgn1,rgn2:THandle;
  hBrush:THandle;
  Rect1,Rect2:TRect;
begin
  PaintBox2.CleanupInstance;
  Rect1.Left:=100;
  Rect1.Top:=200;
  Rect1.Right:=300;
  Rect1.Bottom:=400;
  rgnb:=CreateEllipticRgnIndirect(Rect1);
  rgnt:=CreateEllipticRgnIndirect(Rect1);
  rgn1:=CreateEllipticRgnIndirect(Rect1);
  rgn2:=CreateRectRgn(100,200,300,j);
  CombineRgn(rgnb,rgn1,rgn2,RGN_DIFF);
  CombineRgn(rgnt,rgn1,rgnb,RGN_DIFF);
  CombineRgn(rgn,rgnb,rgnt,RGN_AND);
  //{
  PaintBox2.Canvas.Brush.Color:=clRed;
  if not FillRgn(PaintBox2.Canvas.Handle,rgnb,PaintBox2.Canvas.Brush.Handle) then ShowMessage('rgnb Failed');
  PaintBox2.Canvas.Brush.Color:=clBlue;
  if not FillRgn(PaintBox2.Canvas.Handle,rgnt,PaintBox2.Canvas.Brush.Handle) then ShowMessage('rgnt Failed');
  //}
end;end.