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.

Sam
March 2008