Why are the frames jerking? What SD cards can I use that won't have this problem?
Honestly I would not expect this to happen. If the card is too slow, every camera I've seen it happen with simply stops recording.
In all likelihood, you are seeing dropped frames on playback. This means you computer (or some component of it like the graphics card, chipset, memory or I/O) is too slow. If you are trying to playback the movie directly from the card, then the connection will usually be too slow (unless you use a fast card reader).
There is a way to check: Do you notice the dropped frames when you play the video in-camera? If not, then it is your computer. If yes, then you should try multiple subjects, to see if it happens consistently or not. It may be possible that the encoding used on the camera does not have enough bit rate under certain conditions (movements with lots of fine details for example).
Also, on some Canon cameras you can take a picture DURING video recording. AFAIK, this always results in skipped frames, up to 1 second.
My 5D mark II drops frames too. No computer problem, no card problem. It's processor problem. 7D lost frames too.
In reading many of the blogs on this issue of dropped frames, and relating these to the kind of filming I have been doing I notice that the problem occurs when flying the camera from ambient dark to bright light, like going from outdoors to indoors, or vice versa. As someone suggested, the auto adjustments to ISO seem to create some sort of overload to the camera's systems.
you mean it's skipping frames? If so the camera simply runs out of memory trying to buffer frames as it's shooting them faster than it can write them to the card. Getting a higher speed card may solve that problem, but eventually you will run up against the maximum speed the camera can write at (which I do hope Canon engineers took into account when they designed the camera). Slowing down the frame rate you're shooting at (if you can do that) is also an option, but will of course cause the recording to be a bit jerky if played back at normal speed.