I have found an odd problem with Gnome 3 on Ubuntu recently. Under certain circumstances, when switching between two workspaces the whole desktop and user interface will 'freeze' for anything up to 2 minutes; during this time I cannot click anything or interact using the keyboard. Often, the screen will show a 'half-completed' animation of the workspace swapper.

This is on Ubuntu 12.04.1 running on a 2-year old Macbook Pro, and it only seems to occur when using an external monitor. This does not seem to be a processor or memory issue, and does not occur every time I switch workspaces.

Is this a known problem? What can I do in the short or long term to fix it?