Hi IChan,
thanks for the reply.
1) Users had to change passwords each month and the disconnection happens randomly on any Mac. This user had the problem first so we rejoined the Mac to the network: after this, the userchanged its password for more than 2 months without problems. Yesterday he had the problem again.
2) DNS settings are ok: the DNS server is the only DC in the network and the search field is the domain.
3) Randomly on any Mac.
4) Next time I'll do the centrify restart test: do you need the logs to check further?
Thanks again