tx_run = new Thread(new ThreadStart(SendCanFrames));
if (!tx_run.IsAlive)
{
    running = true;
    tx_run.IsBackground = true;
    tx_run.Start();
}

private void SendCanFrames()
{
    while (running)
    {

    }
}

public void Start()
{
    tx_run.Suspend();
}

public void Stop()
{
    tx_run.Resume();
}

        public void RunTX()
        {   
            tx_run = new Thread(new ThreadStart(SendCanFrames));
            if (!tx_run.IsAlive)
            {
                tx_run.IsBackground = true;
                tx_run.Start();
            }
        }

        public void RunRX()
        {
            rx_run = new Thread(new ThreadStart(ReadCanFrames));
            if (!rx_run.IsAlive)
            {
                rx_run.IsBackground = true;
                rx_run.Start();
            }
        }

private void ReadCanFrames()
        {
            ushort prev_time_stamp = 0;

            while (running)
            {
                if (CanDevice != null)
                    CanDevice.Read(ref rx_can_msg, 1, ref read_cnt);

                if (read_cnt == 1)
                {
                    read_cnt = 0;

                    this.Invoke(
                         (MethodInvoker)delegate
                         {
                             dataGridViewCanRx.Rows[0].Cells[0].Value = rx_can_msg[0].Id.ToString("X");
                             dataGridViewCanRx.Rows[0].Cells[1].Value = rx_can_msg[0].Size.ToString();
                             dataGridViewCanRx.Rows[0].Cells[2].Value = BytesToString(rx_can_msg[0].Data);
                             dataGridViewCanRx.Rows[0].Cells[3].Value = (rx_can_msg[0].TimeStamp - prev_time_stamp).ToString();
                         });

                    prev_time_stamp = rx_can_msg[0].TimeStamp;
                }

                prev_time_stamp = rx_can_msg[0].TimeStamp;
            }
        }

        private void SendCanFrames()
        {
            if (tx_can_msg.Length == 0) return;

            VSCAN_MSG[] l_msgs = new VSCAN_MSG[2];

            while (running)
            {
                for (int i = 0; i < tx_can_msg.Length; i++)
                {
                    if (can_messages[i].CountRun < can_messages[i].CountMax)
                    {
                        can_messages[i].TimeStamp1 = DateTime.Now;
                        interval = can_messages[i].TimeStamp1 - can_messages[i].TimeStamp2;

                        if (interval.TotalMilliseconds >= can_messages[i].Period)
                        {
                            can_messages[i].TimeStamp2 = DateTime.Now;

                            l_msgs[0] = tx_can_msg[i];

                            //send CAN frame
                            CanDevice.Write(l_msgs, 1, ref written_cnt);
                            // send immediately 
                            CanDevice.Flush();

                            can_messages[i].CountRun++;

                            this.Invoke(
                                 (MethodInvoker)delegate
                                 {
                                     dataGridViewCanTx.Rows[i].Cells[4].Value = can_messages[i].CountRun.ToString();
                                 });

                            Thread.Sleep(can_messages[i].Delay);
                        }
                    }
                }
            }
        }

private void buttonStartAll_Click(object sender, EventArgs e)
        {
            int can_channel;
            for (can_channel = 0; can_channel < 4; can_channel++)
            {
                if (started[can_channel] == false)
                {
                    if (connected[can_channel] == true)
                    {
                        switch (can_channel)
                        {
                            case 0:
                                mainform.userControlCan1.Start();
                                mainform.userControlCan1.RunTX();
                                mainform.userControlCan1.RunRX();
                                buttonCan4Start.Text = "STOP";
                                started[can_channel] = true;
                                break;
                            case 1:
                                mainform.userControlCan2.Start();
                                mainform.userControlCan2.RunTX();
                                mainform.userControlCan2.RunRX();
                                buttonCan4Start.Text = "STOP";
                                started[can_channel] = true;
                                break;
                            case 2:
                                mainform.userControlCan3.Start();
                                mainform.userControlCan3.RunTX();
                                mainform.userControlCan3.RunRX();
                                buttonCan4Start.Text = "STOP";
                                started[can_channel] = true;
                                break;
                            case 3:
                                mainform.userControlCan4.Start();
                                mainform.userControlCan4.RunTX();
                                mainform.userControlCan4.RunRX();
                                buttonCan4Start.Text = "STOP";
                                started[can_channel] = true;
                                break;
                        }
                    }
                }
            }
        }