You are probably using an outdated version of Java 1.4.2 for Mac OS X which posed some stability problems with applets in general. Apple seems to have fixed those problems with the recent release of "Java 1.4.2 Update 2". So, please update to "Java 1.4.2 Update 2" using your Mac's software update function. The problems with lock-ups should be gone after updating your Java version.

For more info see http://www.apple.com/support/downloads/javaupdate142.html