[Zope] disappearing external methods

I have recently moved our Zope installation over to a centos server. Since the move all the external methods periodically stop working for no apparent reason and the error is returned as: "external method can not be called because it is none" The thing is that it works well and then stops. Does anyone have any ideas about this? The installation was not initially compiled on centos as I have a large collection of software as part of the application which can be moved between servers with the Zope installation. The reason for the move to Centos is that DTMLTex fails when trying to run pdflatex on Ubuntu and this seems to be distro specific, with no apparent way to fix it, unless anyone knows any better? Kind Regards Garry