[Zope] Zope + Apache + mod_proxy

Hi all, I have a strange connection problem with Zope + Apache + mod_proxy. Sometimes users click on links in their browsers (regardsless which browser: IE 5.5 or Netscape 4.7). The browser waits for data, but nothing comes back. If the user stopps the transfer and click on the link again the transfer will be done immediatly. Has anybody an idea what to do ? Regards, Dirk