[Zope] zope stops responding

Following a hard disk crash 2 weeks ago we have installed Zope onto a new server and all was fine until yesterday morning when Zope stopped responding and required a restart to get it working. It did the same at 3-30pm today. We are using Zope 2.9.0 on Centos 5.1 on a quad Zeon server with 4gb. Ram. At the moment the server is in quite heavy use with teachers trying to write reports for a deadline tomorrow. These are sent to a Postgres DB via psycopg. There is nothing in Z2.log or event.log to point me to the problem. I am therefore asking for advice on the sorts of things that can cause Zope to stop responding and whether there is anything we can do to mitigate against such an event. Thanks and regards Garry