Ubuntu 12.04 CIFS freeze

Ubuntu 12.04 CIFS freeze

I'm running Ubuntu 12.04 I have installed samba and cifs fs type
I'm trying to mount a windows server from Ubuntu.
I have in my fstab a cifs mount that looks like this:
//remote-server/Documents /mounts/remote-server cifs
user=,passwd=,rw,defaults,dir_mask=0777,file_mask=0777
After a few hours I can't execute a df -h. It just hangs.
I have to either: umount -l or reboot the whole server
If I try to ls the mounts directory on the nix machine, it hangs. (not the
sub-mounts, just the list of mount folders). It's bad enough that CTRL-C
or CTRL-Z doesn't work, I have to close the terminal and open a new one.
I'm relatively a newbie for Linux. If you have any questions, let me know
and I'll post it.
Thanks!
-Mike D