Hello,
Could it be a problem related with the architecture? I had the servers installed in a xen environment over amd64 servers (debian etch amd64). I have moved the servers with the same schema, that is, running as vm in xen servers, but over intel processors (debian etch i386) and the problems has gone away.
It is difficult to understand for me such a low-level dependency in openldap or dbd, but with this I has solved our problems.