Ldap freeze and users can't connect

Hi,

I ve got a problem with my ldap network server,
when they are near from 40 people connected
the slapd server can't respond to others andi need to restart it to solve that.

Is it a cache problem? The load average is near 0.6 all the time
On this server i put an nfsd server too

Code:

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include        /etc/ldap/schema/core.schema
include        /etc/ldap/schema/cosine.schema
include        /etc/ldap/schema/nis.schema
include        /etc/ldap/schema/inetorgperson.schema
include        /etc/ldap/schema/samba.schema

include        /etc/ldap/schema/autofs.schema
include        /etc/ldap/schema/cri.schema
include        /etc/ldap/schema/internet2.schema
include        /etc/ldap/schema/supann.schema
include        /etc/ldap/schema/ustl.schema
include        /etc/ldap/schema/rfc2739.schema

pidfile        /var/run/slapd/slapd.pid
argsfile        /var/run/slapd/slapd.args

loglevel        110
logfile /var/log/slapd.log


modulepath      /usr/lib/ldap
moduleload      back_hdb

#sizelimit 2500

tool-threads 1

backend        hdb
database        hdb

suffix          "dc=test,dc=priv,dc=univ,dc=fr"
rootdn          "cn=admin,dc=test,dc=priv,dc=univ,dc=fr"
rootpw {SSHA}XXXXXXXXXXXXXXXXXXXXxxxx
directory      "/var/lib/ldap"


cachesize      100000
#dbconfig set_cachesize 0 4097152 0

#idbconfig set_lk_max_objects 1500
modulepath      /usr/lib/ldap
moduleload      back_hdb

#sizelimit 2500

tool-threads 1

backend        hdb
database        hdb

suffix          "dc=test,dc=priv,dc=univ,dc=fr"
rootdn          "cn=admin,dc=test,dc=priv,dc=univ,dc=fr"
rootpw {SSHA}XXXXXXXXXXXXXXXXXXXX
directory      "/var/lib/ldap"


cachesize      100000
#dbconfig set_cachesize 0 4097152 0

#idbconfig set_lk_max_objects 1500
#dbconfig set_lk_max_locks 1500
#dbconfig set_lk_max_lockers 1500

index objectClass          eq
index cn                    pres,sub,eq
index sn                    pres,sub,eq
index uid                  pres,sub,eq
index displayName          pres,sub,eq
index uidNumber            eq
index uniqueMember          eq
index gidNumber            eq
index memberUID            eq
index sambaSID              eq
index sambaPrimaryGroupSID  eq
index sambaDomainName      eq
index default              sub

#lastmod        on

access to attrs=userPassword,shadowLastChange
        by dn.regex="cn=admin,dc=test,dc=priv,dc=univ,dc=fr" write
        by anonymous auth
        by self write
        by * none

access to *
        by dn.regex="cn=admin,dc=test,dc=priv,dc=univ,dc=fr" write
        by * read

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