Nope. I also don't know what version of AE you are using, what kind of display card you are using, or what kind of machine you are on. All important things to know. The only thing I know is that you are on a Mac and I'm assuming that this happens in the output module settings.
Sorry, yes, it output module settings.
Im on AE CC on a Mac Pro with OSX 10.7.5 with quicktime 7.6.6 and a Quadro FX 4800 and AJA Card. I have the latest drivers for the AJA, and I am not using the AJA AE plugins because they fail miserably if you also run Avid Media Composer.
The first think I would suspect is the AJA drivers.