Apple Quicktime HD trailers and jittery-motion video

[rarredoa]

I watch quicktime trailers directly from apple.com's website. Problem I'm having is that the motion is jittery, just shy of what i would call blu-ray smooth. I have no problem watching blu-ray videos, no matter what codec the movie might be using, AVC, VC-1 or MPEG-2. Is there a particular codec that quicktime uses, I was under the impression that it's AVC for some reason (what audio codec?). Am I the only one who encounters this problem? At 720p, i get flawless video, but 1080p is the culprit. By the way, I let most/if not the entire video to upload onto the quicktime player before i begin to play the trailers. My PC specs are included in my signature, thanks in advance!