Choppy framerate on itunes movie playback

I just downloaded a new movie from itunes, and the playback is really choppy. At first, it worked fine on my laptop, but on an hdtv through an hdmi cable, the frame rate was pretty low, especially noticeable in scenes with lots of camera movement. Playing it in SD seemed to help, but it made the top of the image flicker, and when I first attempted this, I was met with a green screen and no audio. I've deleted and re-downloaded it multiple times, and now, the frame rate is choppy regardless of how I play it. I have another HD movie from itunes that works perfectly fine. I"m running an Asus on Windows 7. Does anyone know why this would be happening? Thanks!