[Zope] Zope Eats Memory

I've been running Zope (with ZEO) on a Debian GNU/Linux box. Just today everything slowed to a crawl and it turned out that one python process was taking up 75% of system memory. Stopping and restarting Zope cured the problem. I'm doing some caching of DTML pages and MySQL queries. Any idea why Zope would start doing this? Are there any preventative measures? Thanks, Chris