关于多线程上载总是有失败的困扰，请高手帮忙看看代码有什么有关问题(100分)

关于多线程下载总是有失败的困扰，请高手帮忙看看代码有什么问题(100分)。
以下是具体的代码实现，请高手看看有没有需要改进的地方。谢谢！

procedure TDownload.Execute;

var

  idHTTP : TIdHTTP;

  isExcetpion:Boolean;

  urlindex:integer;

  url1:string;

begin

  urlindex:=aManager.ThisJob.iUrlIndex;

  iLeft := aManager.arDLFThread[iIndex].iOffsetEnd - iThisBegin;

  aManager.arThisDownloadSize[iIndex] := 0;

    Repeat

     begin

        isExcetpion:= true;

        idHTTP := TidHTTP.Create(nil);

        try

          iThisBegin := aManager.arDLFThread[iIndex].iOffsetBegin + aManager.arDLFThread[iIndex].iOffsetDownload;

          idHTTP.HandleRedirects := True;

          idHTTP.ReadTimeout := 30000;

          idHTTP.Request.UserAgent := 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; Maxthon)';  

          idHTTP.Request.Connection:='Keep-Alive';

          idHTTP.Request.ContentRangeStart := iThisBegin;

          idHTTP.Request.ContentRangeEnd := aManager.arDLFThread[iIndex].iOffsetEnd;

          url1:=aManager.thisJob.iUrls[urlindex];

          idHTTP.DoRequest(hmGet, url1, nil, nil);

          while not aManager.arStop[iIndex] and (iLeft > 0) do

            if iLeft > BufferSize then

            begin

              idHTTP.ReadBuffer(Buffer, BufferSize);

              fs.WriteBuffer(Buffer, BufferSize);

              aManager.arDLFThread[iIndex].iOffsetDownload := aManager.arDLFThread[iIndex].iOffsetDownload + BufferSize;

              aManager.arThisDownloadSize[iIndex] := aManager.arThisDownloadSize[iIndex] + BufferSize;

              iLeft := iLeft - BufferSize;

              aManager.arDLFThread[iIndex].iState := 1;

            end

          else

            begin

              idHTTP.ReadBuffer(Buffer, iLeft);

              fs.WriteBuffer(Buffer, iLeft);

              aManager.arDLFThread[iIndex].iOffsetDownload := aManager.arDLFThread[iIndex].iOffsetDownload + iLeft;