Could the Sun Swing classes cause a significant memory leaks, enough to cause performance issues in an application?
Are there any particular Swing components that cause the garbage collector problems?
Lastly is this a problem with the classes provided by the framework classes or the ineffective use of them?
If the Application is bug enough and the UI complex enough then yes it could cause performance issues. There are no swing components that i am aware of which are, in particular, a cause of memory leaks. But yes the JTable and JEditorPane/JTextPane are the components that you better be careful with. Better always do a cleanup with a destroy() method.
Also the anonymous inner classes sometimes cause issues with garbage collection. So avoid them as far as possible
Lastly the Framework has no issues it is almost always the inefficient use of it that causes issues
Hope that it helps!!