[Samba] winbind stop working after some hours

[Daniele]

Hi, I am trying to use squid proxy with validation on win 2003 active 
directory to filter internet navigation and for it I installed an ubuntu 
10.04 server 64 bit with samba.
My installation looks ok, the server is joined to the AD, ntlm is able 
to validate user, wbinfo report corret information and squid works good.
The problem arise after some hours: winbind become not able to resolv 
info for users and to retrieve info for groups, so squid become not able 
to know id a user belong to a group allowed to navigate and refuse 
connection.
Restarting winbind solve the problem for some hours.
wbinfo report no particular problem; just give back messages like "could 
not get info for user xx" and also setting debuglevel to various numbers 
reports (to me) no significant clues.
I made a workaround scheduling a restart of winbind service at every 
half hour and it works, but is not so elegant ...
Do you have any suggestion to solve this problem?
Thank you
Daniele

samba/winbind version is 3.4.7
squid is 2.7.STABLE7
os is 2.6.32-41-server #88-Ubuntu x86_64 GNU/Linux

smb.conf:
[global]
     workgroup = CED
     realm = CED.AOS
     server string = Samba Server Version %v
     security = ADS
     password server = 172.18.10.24 172.18.10.23
     name resolve order = lmhosts host bcast
     ldap ssl = no
     idmap uid = 15000-25000
     idmap gid = 15000-25000
     winbind separator = +
     winbind enum users = Yes
     winbind enum groups = Yes
     winbind use default domain = Yes
     cups options = raw
[homes]
     comment = Home Directories
     read only = No
     browseable = No
     browsable = No

[printers]
     comment = All Printers
     path = /var/spool/samba
     printable = Yes
     browseable = No
     browsable = No

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