byte[] bytes = new byte[tcpСlient.ReceiveBufferSize];
// Заказываете буфер для чтения неизвестно какого размера -
//  то, что вернет tcpСlient.ReceiveBufferSize тут и то, что он вернет 3мя строками позже не обязательно совпадают

                            while (tcpStream.DataAvailable == true)
// Если данные есть - читаем их, если нет - то не читаем, а выдаем пустой buffer
// (returndata во 2й раз будет не null, а String.Empty)
                            {
                                tcpStream.Read(bytes, 0, (int)tcpСlient.ReceiveBufferSize);
                                returndata = Encoding.UTF8.GetString(bytes);
// Это зачем ???
                                count++;
                            }
                            if (returndata != null)
// Это просто жесть :)
                            {
                                Invoke(rdd, new object[] { bytes });
                                str = returndata;
                                returndata = String.Empty;
                            }

        private void ReceiveRun()
        {
            
            UpdateReceiveDisplayDelegate rdd = new UpdateReceiveDisplayDelegate(Display);
            try
            {
                while (true)
                {
                        if (tcpStream.CanRead)
                        {
                            byte[] bytes = new byte[256];
                            while (tcpStream.DataAvailable == true)
                            {
                                tcpStream.Read(bytes, 0, (int)256);
                                count++;
                            }
                            Invoke(rdd, new object[] { bytes });
                         }
                        else
                        {
                            label1.Text = "Прием невозможен";
                            th.Abort();
                            tcpСlient.Close();
                            tcpStream.Close();
                            return;
                        }
                 }
            }
            catch
            {
                label1.Text = "ошибка";
            }
        }

        delegate void UpdateReceiveDisplayDelegate(Byte [] mydata);
        private void Display(Byte[] mydata) //вывод на форму
        {
            Int32 temp;
            temp = ((mydata[3] << 24) + (mydata[2] << 16) + (mydata[1] << 8) + mydata[0]);
            label1.Text = temp.ToString("D");
            label3.Text = Convert.ToString(count);
                //if (mydata[0] > 0) label2.Text = "OK";
                //else label2.Text = "0";
        }

byte[] bytes = new byte[256];
...

int size = 0;
while(size<256)
{
  while (tcpStream.DataAvailable == true)
   {
     size+=tcpStream.Read(bytes, size, 256-size);
   }
  if (size<256) Thread::Sleep(100);
}
count++;

Invoke(rdd, new object[] { bytes });

if(count_old != count)Invoke(rdd, new object[] { bytes });

       private void button3_Click(object sender, EventArgs e)//Отправить
        {

            if (tcpStream.CanWrite)
            {
                outBuffer[0] = 1;
                if (checkBox1.Checked)
                {
                    outBuffer[1] = 2;
                    outBuffer[2] = 1;
                }
                else
                {
                    outBuffer[1] = 2;
                    outBuffer[2] = 0;
                }
  
                tcpStream.Write(outBuffer, 0, outBuffer.Length);
            }
            else
            {
                label1.Text = "Передача невозможна";
                th.Abort();
                tcpСlient.Close();
                tcpStream.Close();
                return;
            }
        }