I just downloaded Mountain Lion. I'm not sure if it has anything to do with it but recently when I play my timeline in the canvas, weird lagging lines appear across the entire canvas, not just on moving objects like interlaced lines. When I export the video, they're not there. Sometimes it'll go a second or 2 without doing it then all of a sudden these lines appear on the video. They're not physical lines but rather it's as if you're slicing a picture horizontally in two and moving one of the pieces over by a hair.
I'd screen shot it for you but I can't. They only appear when playing. I hope I'm making sense. It's really hard to explain.
I'm usually editing in 1080p Pro Res 23.98 but it's doing it to 720p 60fps stuff too. :(