UDP setup and teardown with application focus and application pause

The app I am writing which uses custom UDP sockets works very well EXCEPT for extreme flakiness when the app is opened and closed and then re-opened.

I'm trying to tear down the socket on ApplicationPause and re-initialize it on ApplicationFocus but seem to run into freezing and/or the app quitting sporadically.

I know I am not doing something right, I just don't know what

When the app comes into the Focus, I am doing

IPEndPoint ipep = new IPEndPoint(IPAddress.Any, port);
newsock= new UdpClient(ipep);

receiveThread = new Thread(new ThreadStart(UDPReceiveData));
receiveThread.IsBackground = true;
receiveThread.Start();

When the app is going to be Pause, I am doing

if (receiveThread!=null)
{
receiveThread.Interrupt();
}

if (newsock!=null)
{
newsock.Close ();
}

Is my thinking correct..... should I being tearing down and re-setting-up like this. I know that IOS let's apps have a certain amount of time before shutting everything down when going into the background. Is perhaps this related to my freezing/quitting problem..... re-opening the app before all background tasks for fully shut down?

Thanks in advance for any help....