My guess would be a faulty catridge - developing a problem with ink flow as it runs. Windows test prints are not usually very long so may not get to the point where the ink gets too faint. I would guess that what you are printing is much longer and so the flow gradually stops. Test the theory by printing a short piece of text of your own, then gradually increasing text length until the problem reappears. Changing the cartridge should cure the problem if my diagnosis is correct, but as I don't use a Canon I can't promise! Good luck. LC.