Waht does it look like in an exported PDF? There are some fonts, Baskerville (at least the version that ships with Macs) for example, that just display badly.
I'm having this Avenir Next Condensed problem too, even in the exported PDF. It's only happening in InDesign, looks fine in Photoshop.
Where did you get this Avenir Next Condensed from?
I'd contact the font maker. Also test in a different version of InDesign if possible.
What is your exact version of InDesign CS6 on what operating system?
( To check the exact version hold Strg (Windows) or Cmd ( Mac OSX ) and invoke menu "About InDesign".