exploradorgt,
I have exactly the same problem as you with my Pixma 7751.
Have you resolved it?
What did you do?
Thanks - Mike
I've found it's a common problem with the IX6810 / IX6820 / IP7210 (yours) and some other models.
No solution so far, been looking for a printhead replacement, sadly these are RARE and the ones I've found are overpriced.
Found some testimonials on the Official Canon Support Forums, a few shared their story on replacing the print head and finding the problem persisted, there was no way to know if the replacement was bad as well.
There was ONE guy saying he got a replacement, same problem, so he put the original bad one back in... and the problem was gone, but came back after a few weeks.
I personally, don't think it's a printhead problem, instead I suspect a logic board issue, perhaps a blown component but the machine is not pretty to tear apart. What I've been doing is printing in "high resolution paper" mode, so the printer uses the other black nozzles and cartridge. It's slow and doesn't get you the same quality results but it's better than nothing.