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It sounds like your printhead ink recepticles have become contaminated. Did you ever have the cartridges not locked down securely -- or do you have a child near your printer? In any event, many ink problems can be solved with ordinary water.
Remove your cartridges and tape over the openings on them while you clean the printhead for about 48 hours. Using warm, but not hot water, and holding the printhead at an angle so the water runs out the front of the head rather than over the contacts on the back, rinse the recepticle ovals ubder a slow, soft stream of water until the recepticle begins to turn white.
Get a shallow, flat plastic container large enough to hold the printhead and a small piece of wood or plastic ( I used plastic Q-tips) to prop the printhead so it is level. Put warm water in the container until it is just below the gold contacts. Add warm water to the inside until the recepticles are covered. Let it sit for a while, then change out the water and do it all over again several times. Once it begins to not discolor the water within an hour, leave it in the tray with fresh water over the recepticles and underneath for an overnight period.
Once you are satisfied that no more ink is disolving, let the printhead dry for 24 hours or put it in front of a fan for several hours until it is completely dry. Then put it back in the printer, reinstall the ink cartridges and run your cleaning and alignment programs until it looks right.
If it doesn't work the first time, go back and do it again, only for a longer period of time. I did it twice before my clogging problem was solved.
The reason I did it was because ordering a replacement printhead from Canon turned out to be nearly impossible. Rather than toss the thing out I had to try this technique out that I got off a internet search. It worked for me.
The Windex treatment is easier, I think, and it works well. Go the DP Review web site and do a search in the "Printers and Printing" forum pages, and you should find the instructions. Or just do a Google search. There are a lot of articles about how to clean your print head on the internet.
Thank you all for taking the time to comment,the problem appears to have gone away for the time being i changed the black cartridge for the third time and all of a sudden the black is nice and shiny but i dont think i have seen the last of it i am thinking i should perhaps have a spare printer head but as yet havent been able to source one