High speed, I never use it. It causes more problems to me than it's worth. Yes, even on the 9900.
Especially high speed and only 720 dpi. Others mileage will vary here. I only use high speed on smallish
prints, 11 x 14 and smaller, and when the media is RC glossy and or luster.
Depending on the matte paper thickness, you almost may want to increase your platen gap to wider, however since you fixed the problem
by backing off to 1440 and Not high speed you fixed it. You may want to try a head alignment, as on the 7900 it's very easy and all
automated.
Paul