[Zope] mysterious periodic crashes

Hi -- I have been having a problem where a Zope server I am running periodically (as in every couple days) crashes whenever a user tries to access an external method. Initially it returns the error "(external method) file does not exist)", and then all the Zope programs terminate, making the website inaccessible. Restarting seems to solve the problem. Does anyone know what might be causing this and how I could prevent it from occurring? Thanks, Matthew Bell Graduate Student University of Pittsburgh