This sounds like it might be a defective CCD imager, and is a known defect for your particular model camera.. If so, Canon should fix this for you for free if you live in the US, Canada, and a select few other countries. If you live outside the US, contact Canon support for your country to see if they'll still honor the advisory (some still will, especially if you stress you knowledge that this is a known defect of the camera). They may also include free shipping both ways (free shipping varies with countries). This is regardless of your camera's warranty status. Please see this link
for more info.
Affected cameras with this problem include:
A60, A70, A75, A80, A85, A95, A300, A310, S1 IS, S60, S400, S410, S500, SD100, SD110, IXUS V2/V3/300/400/430/500/II/IIs, IXY Digital 200a/300a/320/400/450/500/30/30a