Bite the bullet, buy a new cartridge, and see if the printing returns. If so, then that was the problem.
There are aftermarket places which will refill your cartridges and "reset" the chip. There's a place here in our local Mall with a pink pig as a logo, but I can't remember the name. I just buy cartridges at Sam's Club.
I think we have a winner. I had bought a new cartridge and it didn't even last a week. I thought it was just bad and refilled the old one with ink. If this is true then both were EXPIRED. If that's the case, I'll get kicked off if I posted what I'm thinking about HP right now.