I tried running the instance on a single thread as suggested by a how-to I found at http://zope.org/Members/matt/StabilityHOWTO This has "fixed" this problem and also another long running problem with a huge memory leak. The howto suggests this indicates a problem with a C -extension. Is there any quick way to track this down or am I stuck with removing products out of the equation until I find the culprit? Is it definite that it will be a problem with a Product or could it be elsewhere in zope/python? Answering a couple of other questions...

What OS are you running on?

Debian stable

...why aren't you running on Python 2.3?

Currently running zope 2.6.2 but in the middle of upgrading dev server to zope 2.7 and python 2.3 to see if this helps. Thanks for your help with this. Richard.