thanks for your replay. I don't know what is an XiD, and after a series of trial I removed the nvidia drivers, otherwise I was not able to use the machine at all. Then, after some problems, I had to reinstall the complet OS, so all the logs are not recoverable any more.
Anyhow I've read about some other guys experiencing the same problem in another thread, and maybe it could depend on some bug in the driver itself.
If you think that XiD could give us insights on the problem, I can reinstall the driver and let the system freeze once more.
Thank you again,