Sorry, I can't help you solve this, but just to report that I am having the same problem with a MG5550, and many other people seem to be reporting the same. In my case, lines seem to be printed e.g. two lines bold, then two not bold, three lines bold, two not bold etc. - not quite a fixed pattern, but not completely random. The sheet I have just printed has about half the lines printed in bold, in total. I have just printed a document from a different computer, and this has the same problem with my MG5550, so it doesn't seem to be a problem with just my computer or drivers. I have printed the document on a different printer, and it is printed fine, so it does not seem to be a problem with my computer or software. I get the same problem no matter what font I use, and when the change of font caused different numbers of words per line, it was still the complete lines that were printed in bold. So it seems to be a problem with the way the printer is building each line. I have tried cleaning, deep-cleaning, auto head alignment, and roller cleaning, and have checked the timing strip for grease (it was clean, and in any case had not been touched by anyone). Nothing helped. I was hoping that someone from Canon could explain what mechanism could be causing this.
