Java causes PC to freezes on Vista

We have a new website which incudes a Java drawing application on one page. We have no problems on PC's running XP but we have found PC's with Vista have started to lock up/freeze on this page. This seems to happen after the page has been viewed a few times, and also seems to effect IE and FF (although we have not been able to test all versions).
It seems to be a Java/Vista problem, as no other pages of the site cause problems (only the page containing the Java App.
The problem page is at http://www.heartsandhormones.com/heart/create.
We no very little about Java (the Java application was developed by another person, who hasn't had this problem before).
Is this a know issue? (Googling doesn't seem to indicate this)
Any ideas what this could be?
Any help/advice gratefully received