Comment 38 for bug 1292830

I've encountered the same problem after upgrading to Ubuntu 14.04 on a Macbook Pro "Core i5" 2.53 with a GeForce GT 330M/PCIe/SSE2 graphics card with the NVIDIA binary driver 331.38.

I've tried out the solution recommended on the ArchWiki for what appears to be the same bug:

and since then the problem has disappeared!

NOTE the config files they mentioned do not exist under the same names in the Ubuntu system. While upgrading. NVIDIA would have created an xorg.conf.<dateOfUpgrade> file in /etc/X11/ and this is the one it uses for interacting with XOrg. There is also an an xorg.conf.failsafe file for when there's some errors using the first one. Making the changes described in the ArchWiki article in to BOTH files solved this problem for me.