There is a know graphics problem in version 6 on some Mac OS X systems (usually laptops) that causes this blocking of text. It is an issue between the QT graphics library we use and the OS X system. You should be able to cure the problem by turning off the "Font Smoothing" in the Appearance pane of the System Preferences.
There has been some change in where this setting is located and whether or not it takes effect immediately. On recent OS versions, it may be located in the General pane, and you may have to restart the computer in order for the changes take effect.