I am having the same choppy video problem. I have tried all the fixes mentioned here and all the ones suggested by Apple and nothing has worked so far.
The annoying thing is, up until last week, it was fine. I download a TV show every Tuesday and last week, before downloading it asked to do an update which I duly did. Had loads of problems getting the show to download after that so, when it was glitchy I just chalked it up to a one off. But it's the same again today. This has ONLY happened since I installed Quicktime (which I didn't previously have installed) and updated Itunes to version 11.0.
I've uninstalled Quicktime but that hasn't solved the problem.
My system requirements are fine, my settings are fine, I have checked and double checked everything. Nothing fixes it. I have a couple of other things I might try before I admit defeat but it's looking like I've wasted money on a pre-ordered series I won't be able to watch. Even if I do manage to fix this, I'm never buying another itunes product again.
If anyone else out there has a ny other suggestions on how to fix this problem I am open to hear them.