There have been a variety of causes discussed to explain pan-judder (shutter angle, the AVCHD spec itself, rolling shutter, or simply the guy panning too fast cos of his inabilities! Loved that one). I want to point out another cause which certainly CAN occur (happened to me on a multi-camera shoot). If your editor is set to a different frame rate from the camera, then it will perform a crude type of frame rate conversion; essentially it will either drop or duplicate frames to force it in. So if you accidentally film at 30fps, but your editor timeline is set up to 24 or 25 fps, the editor will drop roughly every 6th frame when you import.
You won’t even notice this on stationary scenes or just dialogue, but it will really jump out at you when you…… PAN.