There is a known bug in this release that makes it difficult to switch away from the default 331.89 drivers provided as stock drivers.

I got around this bug by editing grub to boot to command line,

then purge nvidia*,
copy blacklist.nouveau.conf to /etc/modprobe.d,
reboot,
install nvidia drivers manually (340.46),
reboot,
use vi to remove changes (or save original file in a backup file and copy to the active grub file) and update grub.
reboot.

The launchpad LoctusofBorg 7.4.23 seems to work better than the stock 7.4.8