I've had this HP Deskjet 990Cxi for years, and took it out of storage to use for printing mailing addresses on card stock. The cards I'm printing are 6" x 11.5", and I'm using an A4 (8" x 11.6") setting as that seems to be the best approximation regarding length of the card.
The first sheet card comes out fine, the second sheet prints fine, but upon ejecting, about half of the third sheet follows then jams, with the third sheet halfway out and no addresses printed on the third sheet. I can usually press the feed button and it will eject the third card sheet and eject a 4th card sheet too. Then it will print two more cards just fine and repeat the jam by pushing a 3rd sheet out halfway with the 2nd sheet that was printed.
I've used rubbing alcohol to clean the rollers, I've tried different media settings for thickness, gloss, and even tried standard letter size, and it still sucks up the third sheet early and partially ejects it with the second sheet.
It prints out perfectly fine with standard 20# letter stock paper.