Yes, I use JSTARS all the time. No problems at all and it's very stable. Do you have the latest JAVA runtime installed? Are you running the registered version or the free version?
Also, are you using the latest version of JSTARS?
http://jstars.altervista.org/Make sure that you 'Disconnect' JSTARS before exiting RadarBox, as it can get confused when it's looking for RB port output that isn't there.
What version of RB are you running?
Another thought, are you using more than one add-on that uses the port 30003 data output? I seem to remember that early versions of RB wouldn't accept more than one connection at the same time.