debugging gui/event/threading problems

I have a swing applet that regularly does screwy things in the gui.
Text caret focus goes and can't be regained. messed up painting. This
is just simple JTextPanes and StyledDocuments. I have my own threads
that update the gui through calls to the documents, synchronising on
the document and calling invokeLater.

What is a good way to track down problems such as these?

Is there anything wrong with this method, called from a user thread?

   private void appendInText(final String owner, final String text) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            Logger.log("appendInText(): " + owner + ", " + text);

            final AttributeSet attributes;
            if (user.equals(owner)) attributes = userAttr;
            else if (recipient.equals(owner)) attributes = reciAttr;
            else attributes = consAttr;

            final StringBuffer builder = new StringBuffer();
            builder.append(owner);
            builder.append(": ");
            synchronized(inDocument) { // explicitly thread safe
               final int length = inDocument.getLength(); //
StyledDocument
               if (length != 0) builder.insert(0, '\n');
               try {
                  inDocument.insertString(length, builder.toString(),
consAttr);
                  inDocument.insertString(
                     length + builder.length(), text, attributes);
               }
               catch(BadLocationException ex) {
                    Logger.log(ex);
               }
            }
         }
      });
   }

TIA ,
Mike W