[Zope] Problem with undo

Hi, since I installed the newest-stable Zope version (2.5.1) under Linux Susse 7.3, I have had problems when I click on the "undo" tab of the "manage interface". Sometimes it takes a long time to show me all the transactions that I can undo. On the while, it blocks Zope. So, people who is visiting the site can't see it till this is done. Often they will see the common error message that browsers show when they can't load the page. Is there any way of solve this problem? What are the causes? Thanks in advanced, Josef.