On these Canon printers coarse dithering is the classic sign of slightly incorrect paper feed.
If you haven't already done so, do a manual banding adjustment for every paper you use. It requires two sheet of paper (even A4 will do), and you must use the same paper on wich you are going to print. Note that the adjustment is per media type: this means that you have to repeat it for every media type you use. Also, since the correct settings are different for different papers, you can't use the same media type to print on more than one paper.
If that doesn't solve the problem, do an automatic head aligment. You need to do this only once, but use an high quality glossy media.
Finally, if nothing improves, have your printer checked by Canon. There are reports of mechanical issues that can cause the sort of trouble you are facing.