(Buggy) JLabels

You're still calling swing methods from a non-EDT. Line 55 you create a new Thread with 'this' as the runnable, in the "run" method it calls isVisible() which is a Swing/AWT method. I'm suspicious that your kind isn't thread-safe generally. I really don't think you should be messing with threads, to be honest. They may seem simple but they are actually so difficult to get right.

Either re-write your program so you don't need to create new threads, or fix all the uses of Swing/AWT methods so that they occur on the event thread (i.e. use EventQueue.invokeLater or similar as appropriate).