Bug with desaturate highlights filter? It seems to pixelate whenever I'm rendering to in high precision YUV.. Not sure if this is fixed in the 6.0.5 update.
I'm not working off similar clips or codec, but as far as i can try with downloaded stills converted to 720P QT movies, i can't seem to duplicate it on an intel quad running FCP 6.0.4.
However, there was a post on it a while ago on the cow.