Windows Phone定时器间隔不一致

我希望使用c#创建一个windows phone应用程序。我想有一个计时器，显示一个图像100毫秒，然后切换到另一个图像，然后等待另一个900毫秒之前，它再次闪烁图像。我已经写了下面的代码，然而，它似乎不始终闪烁。什么好主意吗?

public partial class MainPage : PhoneApplicationPage
{
    DispatcherTimer timer = new DispatcherTimer();
    List<string> files = new List<string>() { "Images/off-light.png", "Images/on-light.png" };
    List<BitmapImage> images = new List<BitmapImage>();
    int current = 0;
    // Constructor
    public MainPage()
    {
        InitializeComponent();
        foreach (string file in files)
        {
            BitmapImage image = new BitmapImage(new Uri(file, UriKind.Relative));
            images.Add(image);
        }

            timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromMilliseconds(900);
            timer.Tick += new EventHandler(timer_Tick);
    }
    void timer_Tick(object sender, EventArgs e)
    {
        myImage.Source = images[current];
        current++;
        if (current >= files.Count)
        {
            current = 0;
            timer.Interval = TimeSpan.FromMilliseconds(100);
            timer.Stop();
            timer.Start();
        }
        else
        {
            timer.Interval = TimeSpan.FromMilliseconds(900);
            timer.Stop();
            timer.Start();
        }
    }
    private void btnStop_Click(object sender, RoutedEventArgs e)
    {
        timer.Stop();
        myImage.Source = images[0];
    }
    private void btnStart_Click(object sender, RoutedEventArgs e)
    {
        timer.Start();
    }
}