The current Frames per Second are displayed in the lower right corner of the Game. The Game runs really choppy. Also, make sure you don't have another Browser Window open that has a Java-Applet running. Make sure you get the latest Java Virtual Machine from Microsoft's Web-Site. That will, most likely, solve the Problem.
Computer locks up when trying to play sound.
There is a Problem within Java that sometimes causes the VM to crash if another program is playing a sound while Java try's to access the Sound-Hardware. Turn off programs like ICQ, that could interrupt you with a Sound-Message while you are playing. The Graphics are all messed up. The Game doesn't start at all. Make sure you are not behind a Firewall.