Ubuntu 12.04 LTS is stuck on Samba 3.6.3 which has a lot of bugs. A lot of these have already been solved by the Samba development team, but the new releases are not available in the Ubuntu 12.04 LTS repository.
To update Samba, add the nathan-renniewaldock PPA to your system by entering these commands :
sudo add-apt-repository ppa:nathan-renniewaldock/ppa
sudo apt-get update
Now you can update Samba to version 3.6.7 (especially Windows XP winbind issues which cause Samba crashes)