JFrame *sometimes* updating text on JLabel

I am building an application that has the same functionality as Gmail
Notifier using Java Swing components. The application starts by
showing a login jframe where the user enters username, password, etc.
and then hits a "connect" button. Then I want to pop up a message box
that says "connecting to the server...". In the background, an http
request is fired off and when the app receives a response from the
server, the message pop-up will disappear.

I have implemented the pop-up box as a singleton JFrame class that has
a single panel with a JLabel. When the user hits connect, I use setText
to set the "connecting..." message, then do a setVisible to display the
message box.

The problem is that when the pop-up box appears, only *sometimes* does
the "connecting to server..." text appear on it, the other times, a
blank window shows up with no text.

I assume I am missing something to do with setting the visibility, the
focus, or repainting the frame, but I am confused that it *sometimes*
updates properly. But I am wondering if it could also be an environment
problem with my IDE (Eclipse 3.1.2), older Java version (compiling with
1.3), or the multi-threaded nature of the app with all the Timers going
off.

Here is the code for the message pop-up (condensed where possible)
class:
**********************************
public class SystemMessageBox extends JFrame {
    JLabel systemMessageLabel;
    JPanel mainPanel, buttonPanel;

    // Singleton pattern
    private static class SystemMessageBoxHolder {
        private static SystemMessageBox instance = new SystemMessageBox();
    }

    public static SystemMessageBox getInstance() {
        return SystemMessageBoxHolder.instance;
    }

    private SystemMessageBox() {
        super("CLC2S RRTS+ Notifier");
        this.setVisible(false);
        create();
        this.setResizable(false);
        setIconImage(NotifierUtilMethods
                .loadImage(NotifierConstants.CLC2S_LOGO_ICON));
    }

    public void create() {

        // Create the panel to hold request info
        JPanel messagePanel = new JPanel();

        // Create main panel
        mainPanel = new JPanel();
        mainPanel.setLayout(new BoxLayout(mainPanel, BoxLayout.Y_AXIS));
        mainPanel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
        mainPanel.add(messagePanel);

        // Add panel for labels
        systemMessageLabel = new JLabel();
        systemMessageLabel.setHorizontalTextPosition(JLabel.CENTER);
        messagePanel.add(systemMessageLabel);

        setContentPane(mainPanel);
        addWindowListener(new CloseWindowListener());
        pack();
    }

    public void updatePopUp(String message) {

        // updates message
        systemMessageLabel.setText(message);

        pack();
        SystemMessageBox.getInstance().repaint();
        this.requestFocusInWindow();
        SystemMessageBox.getInstance().setVisible(true);
    }
}
**********************************

(*In the update method, I have been throwing the kitchen sink at the
problem, so I realize there could be some redundancy in termns of
repainting, requesting focus, setting visibility, etc.)

Again, sometimes the JLabel updates correctly, other times, a blank
window shows up.

Any ideas, thoughts, or comments?
Thanks.