Several years ago, a large batch of defective Sony CCD imagers were installed in cameras from many vendors, including Canon. This occurred over the space of a couple years, and involved literally millions of cameras. The sensors will degrade with age, eventually resulting in what you're experiencing. There are also many other symptoms that may result from these defective CCD imagers (black pictures, purple pictures, over-exposed pictures, lines in pictures...).
Kudos to Canon in that they fully stand behind their products. If your camera is experiencing CCD failure, Canon should fix this for you for free, including free shipping both ways. This is regardless of your camera's warranty status. Please check the below "Related Link" link for more info.
Applicable cameras include:
A40, A60, A70, A75, A80, A85, A95, A300, A310, S1 IS, S60, S200, S230, S330, S400, S410, S500, SD100, SD110, IXUS V2/300/400/430/500, IXY Digital 30/30a/200a/300a/D320/400/ 450/500
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