Disabling compiz seems to have fixed the freezing.
There might be a shorter way to do this, but this is what I've done.:
In Ubuntu Software Center find and install Compiz Fusion Icon.
Run it:
Applications -> System Tools -> Compiz Fusion Icon.
Then on the blue icon that appears, right click, go to Select Window Manager, and choose Metacity.
Disabling compiz seems to have fixed the freezing.
There might be a shorter way to do this, but this is what I've done.:
In Ubuntu Software Center find and install Compiz Fusion Icon.
Run it:
Applications -> System Tools -> Compiz Fusion Icon.
Then on the blue icon that appears, right click, go to Select Window Manager, and choose Metacity.