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).
Actually, I'm not sure if this xen stuff was ever resolved: http://www.openldap.org/lists/openldap-software/200603/msg00214.html
I'd be interested in an update on any other xen experiences people have had. (you didn't mention if you were using linux, netbsd, or some other xen-support arch)