If you are using the High Definition (HD) video setting, you "must" use a high speed SD memory card. A regular SD memory card will drop frames when you are recording because the card can not keep up with the camera.
When playing back videos shot with the SX20IS, I sometimes see horizontal lines across the picture, especially when the camera was panning. This happens more when I watch the video on a computer than on the camera viewfinder. This didn't happen with my previous Canon. Any ideas on what is causing this? Thanks.