Hello,
After some tests with openldap, we have studied the only difference between production and new servers: the new ones are installed as a vm in a xen virtualized server. So we have installed a new server (as a real server, without xen) and it seems to work with the production load. So we are thinking now that it could be a network performance problem related with xen (although we have other heavily load servers running under xen without any apparent problem).