et voilà à la rescousse ....
How much ram in that iMac? Running osx and LW confortably needs at least 256MB ram.
Try reinstalling JAVA 1.4.2 update 2 downloadable at
http://www.apple.com/downloads/macosx/apple/
If that doesn't work tell us. I'm using the same iMac (however much modified
) with 512MB ram and no problems running 10 programs at once.
Bonne chance