I have a problem with my novel installation of Ubuntu. Sometimes, the system crashes: the monitor still show the desktop environment, but it is frozen or blocked and the only think that I can do is to press the power button and shot down.

It there a way to understand which kind of problem is? Driver Video, perhaps? How can I do a better diagnosis?

The system is Ubuntu Precise, installed on a Dell XPS 14 Ultrabook, whith Sputinik kernel and Bumblebee drivers for the Nvidia card.

Thanks!!!