in the stop() method tmp.interrupt() throws a SecurityException in PingSender and PingWaiter when running in an applet. I don't believe that tmp.interrupt() call is necessary because you are already assigning null to 't' and in the run() method in the while loop you are checking to see if 't' is not null. As an alternative you could put a try/catch around the interrupt to catch SecurityException and ignore the error.

I've got the code commented out; and that has solved my issues.