The problem may lie with the CCD censor. Free repair by Canon as the Sony chip is faulty. Quite a few models affected. See links:
http://www.imaging-resource.com/badccds.html
Steve W
January 2010
There is a fault with it.
Mine does the same thing. You need to contact Canon and tell them, and they will then give you an address of a company in your area. You send them the camera (you have to pay the postage and packaging etc) and they fix it free of charge and send it back.