Parsing a page's source code for URLs

My code has to download the source code of a page and parse it for URLs. I want it to ask for number which is increased inside the critical section. My problem happens on thread termination.

Main form code:

    unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, OverbyteIcsWndControl, OverbyteIcsHttpProt, StdCtrls,Unit2, Spin;
const
  WM_DATA_IN_BUF = WM_APP + 1000;

type
  TForm1 = class(TForm)
    HttpCli1: THttpCli;
    Button1: TButton;
    ListBox1: TListBox;
    Memo1: TMemo;
    Button2: TButton;
    SpinEdit1: TSpinEdit;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    FStringSectInit: boolean;
    FGoogle: array [0..2] of TGoogle;
    FStringBuf: TStringList;
    FLink:integer;
    procedure HandleNewData(var Message: TMessage); message WM_DATA_IN_BUF;
  public
     StringSection: TRTLCriticalSection;
    property StringBuf: TStringList read FStringBuf write FStringBuf;
    property Link: integer read FLink write FLink;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var i:integer;
begin
  if not FStringSectInit then
  begin
    form1.FLink:=0;
    InitializeCriticalSection(StringSection);
    FStringBuf := TStringList.Create;
    FStringSectInit := true;
     for i:=0 to 2 do
     begin
    FGoogle[i]:= TGoogle.Create(true);
    SetThreadPriority(FGoogle[i].Handle, THREAD_PRIORITY_BELOW_NORMAL);
    FGoogle[i].Resume;
    end;
    end;
    end;





procedure TForm1.HandleNewData(var Message: TMessage);
var k,i,s:integer;

begin
  if FStringSectInit then
  begin
    EnterCriticalSection(StringSection);
    s:=flink;
    inc(s,8);
    flink:=s;

    memo1.Lines.Add(FStringBuf.Text);

    FStringBuf.Clear;
    LeaveCriticalSection(StringSection);
    {Now trim the Result Memo.}
  end;
  if form1.Memo1.Lines.Count>20 then
for k:=0 to 2 do
  begin

  fgoogle[k].Terminate;
  fgoogle[k].WaitFor;
  fgoogle[k].Free;
  FStringBuf.Free;
  DeleteCriticalSection(StringSection);
  FStringSectInit := false;
  memo1.Lines.Add('Thread is done: ' + inttostr(k));

   end;
   end;

procedure TForm1.Button2Click(Sender: TObject);
begin
listbox1.Clear;
end;

end.

Worker thread code:

    unit Unit2;

interface

uses
  Classes,Windows,IDHTTP, OverbyteIcsWndControl, StdCtrls,OverbyteIcsHttpProt,SysUtils,Dialogs;

type
  TGoogle = class(TThread)
  private
google:TStringList;
    Upit:string;
    Broj:integer;
    Buffer : TStringList;
    httpcli1:THTTPcli;
  protected
    procedure parsegoogleapi;
    procedure SkiniSors;
    procedure Execute; override;
  public
    property StartNum: integer read Broj write Broj;
  end;

implementation
uses unit1,StrUtils;

function ExtractText(const Str, Delim1, Delim2: string; PosStart: integer; var PosEnd: integer): string;
var
 pos1, pos2: integer;
begin
    Result := '';
    pos1 := PosEx(Delim1, Str, PosStart);
    if pos1 > 0 then
    begin
    pos2 := PosEx(Delim2, Str, pos1 + Length(Delim1));
    if pos2 > 0 then
    begin
      PosEnd := pos2 + Length(Delim2);
      Result := Copy(Str, pos1 + Length(Delim1), pos2 - (pos1 + Length(Delim1)));
end;
end;
end;
function ChangeString(const Value: string; replace:string): string;
var i: Integer;
begin
    Result := '';
    for i := 1 to Length(Value) do
    if Value[i] = ' ' then
    Result := Result + replace
    else
    Result := Result + Value[i]
end;

(*Ovo je procedura za skidanje sorsa*)

procedure TGoogle.SkiniSors;
var
criter:string;

begin
HttpCli1:=THttpCli.Create(nil);
google:=TStringList.Create;
criter:= ChangeString(Upit,'%20');

With HttpCli1 do begin
    URL            := 'http://ajax.googleapis.com/ajax/services/search/web?v=1.0&start=' + inttostr(broj) + '&rsz=large&q=rocksongs';
    RequestVer     := '1.1';
    Connection := 'Keep-Alive';
    RcvdStream := TMemoryStream.Create;
    try
        Get;
    except
        RcvdStream.Free;
        Exit;
        (*How can I terminate thread here if I get error*)
    end;
RcvdStream.Seek(0,0);
google.LoadFromStream(RcvdStream);
RcvdStream.Free;
ParseGoogleApi;
end;
end;

procedure TGoogle.ParseGoogleApi;
var Pos: integer;
    sText: string;
begin
Buffer:= TStringList.Create;
  sText := ExtractText(google.Text, '"url":"', '","visibleUrl"', 1, Pos);
  while sText <> '' do
  begin
    buffer.Add(sText);
        sText := ExtractText(google.Text, '"url":"', '","visibleUrl"', Pos, Pos);
  end;
  google.Clear;
end;


procedure TGoogle.Execute;
var i:integer;
begin
 while not terminated do
  begin
  EnterCriticalSection(Form1.StringSection);
  Broj:=form1.Link;
  skinisors;

  Form1.StringBuf.Add(buffer.Text);
  LeaveCriticalSection(Form1.StringSection);
  PostMessage(Form1.Handle, WM_DATA_IN_BUF, 0, 0);

  end;
Google.Free;
Buffer.Free;
httpcli1.Free;
end;
end.

Also, how do I deal with timeouts with THttpCli1? Is it a smart idea to use Timer inside threads?