Hey Jay - the 3880 doesn't use vacuum suction at all, so you shouldn't have any trouble. I think it starts with the 4k series...or anything that feeds straight down.
Jan, I reduced the vacuum to the min setting (-4) and still had the same trouble. I think part of the pressure applied to the vacuum manifold comes from the feed rollers themselves, which you can't loosen programmatically.
I've found that any regular backer sheet wreaks havoc in the 7900 (printing arm rips the paper nearly in half, can't be good for the printhead). I'm getting some "Darice perforated plastic canvas" on Amazon to see if it's thin enough to run as a backer on the 7900. Fingers crossed.