Thread.Sleep freeze the UI

I know, it's been here many times, but I still don't know how to do it.

I want to create a program that will repeatedly download data from database, so the user will see the data in the program to this time.

I do not need to load the database quickly, so I use sleep, but sleep freeze whole UI.

I take simple example.

<Window x:Class="ThreadingPrimeNumberSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Prime Numbers" Width="760" Height="500">
    <Grid>
        <Button Content="Start"  
            Click="StartOrStop"
            x:Name="startStopButton"
            Margin="10,10,693,433"
            />
        <TextBlock Margin="87,15,547,424"><Run Text="Biggest Prime Found:"/><InlineUIContainer>

            </InlineUIContainer></TextBlock>
        <TextBlock x:Name="bigPrime" Margin="222,15,409,428"><Run Text="3"/></TextBlock>
        <TextBox Height="104" TextWrapping="Wrap" Text="TextBox" Width="522" Margin="87,295,143,70"/>
    </Grid>
</Window>


...usings...

namespace ThreadingPrimeNumberSample
{

    public partial class MainWindow : Window
    {
        public delegate void NextPrimeDelegate();

        //Current number to check 

        private long num = 3;

        private bool continueCalculating = false;

        public MainWindow()
            : base()
        {
            InitializeComponent();
        }

        private void StartOrStop(object sender, EventArgs e)
        {
            if (continueCalculating)
            {
                continueCalculating = false;
                startStopButton.Content = "Resume";
            }
            else
            {
                continueCalculating = true;
                startStopButton.Content = "Stop";
                startStopButton.Dispatcher.BeginInvoke(
                    DispatcherPriority.Normal,
                    new NextPrimeDelegate(CheckNextNumber));
            }
        }


        public void CheckNextNumber()
        {

            // Reset flag.
            NotAPrime = false;

            for (long i = 3; i <= Math.Sqrt(num); i++)
            {
                if (num % i == 0)
                {
                    // Set not a prime flag to true.
                    NotAPrime = true;
                    break;
                }
            }

            // If a prime number.
            if (!NotAPrime)
            {
                bigPrime.Text = num.ToString();
            }

            num += 2;

            Thread.Sleep(500);



            if (continueCalculating)
            {
                startStopButton.Dispatcher.BeginInvoke(
                    System.Windows.Threading.DispatcherPriority.SystemIdle,
                    new NextPrimeDelegate(this.CheckNextNumber));
            }
        }


        private bool NotAPrime = false;


        }


    }

If was process started and I writing to the textbox or doing something... whole is it freezes.

How should this code look to be able to run the process and UI won't be frozen?