Applet freezes (seemingly) randomly upon certain actions

Question

Hello all,
I am developing a signed java applet for my company in which the user can hit multiple HTML buttons, and those buttons trigger javascript which in turn signals the applet to perform some task. I can't post the source code unfortunately, but the main gist is that the following buttons are available: Upload Files, Preview File, Edit File, Copy To USB, Delete, and Exit.
This applet is used only on a closed system (i.e. we can trust our users 100%), so everything is handled through AccessController.doPriveleged calls, so as to give full access to the actions called from the javascript.
To clarify each of the actions:
(We deal mostly with ppt/pps)
Upload Files - opens up a JFileChooser window for the user to select multiple files. Upon hitting OK, the files are sent to a server for permanent storage, and copied to a desktop folder for temporary storage.
Preview File - Converts a ppt to pps (if needed), and launches it using the Desktop.open() call.
Edit File - Converts file from pps to ppt (if needed) and calls Desktop.open().
Copy To USB - opens a JFileChooser for the user to select a USB stick directory, and copies all files from the desktop to it.
Delete - Displays a confirmation window, and upon hitting OK deletes the file from the client and server.
Exit - Confirms with the user, and upon hitting OK, re-uploads any edited files to the server and navigates away from the page.
Most of these actions are handled through threads. So every time a button is hit, a new thread is spawned. The threads go away once the action is performed in full .. i.e. when all files are uploaded, or a file is launched.
Everything works like a charm, except (seemingly) randomly, when hitting the Upload, Copy, or Delete button the applet will freeze. I have many System.err.println statements throughout the entire application and it seems as though the applet freezes very early on in performing action. Namely, the last debug statement I see before the console freezes is usually "Perfoming action ('action name')" or "Dispatching to privileged thread". The first statement happens right after the doPrivileged call, and the second statement happens right before the call.
There are no exceptions or anything displayed in the java console. This only happens on those 3 actions. The only thing I think they have in common is that they each show a JOptionPane/JFileChooser. The other actions don't. While the applet appears frozen, I noticed I could still launch files for previewing and editing, but no debug output is produced where it should be.
Like I said, this appears random, but happens often. I can sit there and hit preview/edit over and over again for 10 mins and it will never freeze up. However, if I sit there hitting Upload/Copy/Delete over and over, it will normally freeze after 1-10 times (usually closer to 1-5).
I know there is limited help you guys can give without seeing the source code, but I was wondering if anything pops out as a possible cause.
Thanks for reading.
Bryan

Answer 1

Thanks for the response.
I converted to JApplet, and am still noticing this behavior.
Also, this freezing happens regardless of if I actually upload a file or not. What I am doing is just clicking Upload, then cancel, Upload, then cancel, and repeating a few times until it locks up. Same with the Copy button. I don't actually hit OK to copy, just keep hitting cancel. So each time I hit the button the following should be happening:
Button is hit
Javascript function is called
Javascript calls java function dispatchAction.
java function dispatchAction performs a doPrivileged, which inside the PrivilegedAction calls performAction
performAction creates a Worker class, which is a custom class I made that implements Runnable. The reason for this is I had to be able to pass an external variable into the thread, and wasn't able to with a normal Runnable.
the Worker class is run as a new thread, and inside it calls a function that creates the JFileChooser
That's about it. I have been over the code a few times and find this very peculiar, because VERY little happens when the Upload/Copy/Delete buttons are clicked, then cancelled. The only thing I can see is an issue with calling too many doPrivilege or something, or too many threads .. no idea.
Any other ideas?